How to Migrate from Slack to Mattermost in One Weekend

Slack just increased prices again. Your 50-person team now pays $22,500 annually for Business+ tier. Meanwhile, Mattermost runs on a $40/month VPS serving unlimited users.

The migration seems daunting. You have 3 years of conversation history, 47 channels, custom integrations, and a team that resists change. But companies complete this migration every weekend with zero data loss.

This guide walks you through the entire process, from Friday afternoon planning to Monday morning launch.

Why Teams Are Leaving Slack for Mattermost

Before diving into logistics, understand what you're gaining:

Cost savings:

Slack Business+: $15/user/month × 50 users = $9,000/year
Mattermost self-hosted: $480/year (VPS) + $120/year (backup storage) = $600/year
Annual savings: $8,400 (93% reduction)

Data sovereignty:

All messages stored on your infrastructure
No third-party AI training on your conversations
GDPR/HIPAA compliance simplified
Full audit trail control

Feature parity:

Channels, threads, direct messages
File sharing and search
Custom integrations and webhooks
Mobile apps (iOS/Android)
SSO and LDAP support

The only thing you lose is Slack's vendor lock-in.

Friday Afternoon: Pre-Migration Setup (3-4 hours)

Step 1: Export Slack Data (30 minutes)

For Slack Standard/Plus plans:

Go to https://my.slack.com/services/export
Select date range (typically "All time")
Click "Start Export"
Download the ZIP file when ready (5-30 minutes depending on size)

For Slack Free tier: Slack doesn't provide full export. Use third-party tools:

slack-export-viewer (open source)
Manual export of critical channels
Accept that recent 90-day history limitation exists

Export contains:

Channel messages and threads
Direct messages
File attachments (up to storage limits)
User profiles
Custom emoji

Step 2: Deploy Mattermost (45 minutes)

Option A: Quick VPS Deployment

# SSH into your Ubuntu 22.04 VPS
ssh root@your-server-ip

# Install Docker
apt update && apt upgrade -y
apt install -y docker.io docker-compose

# Clone Mattermost Docker deployment
cd /opt
git clone https://github.com/mattermost/docker
cd docker

# Configure environment
cp env.example .env
nano .env

Critical .env settings:

DOMAIN=chat.yourcompany.com
MATTERMOST_HTTPS_PORT=443
MATTERMOST_ENABLE_SSL=true
TZ=America/New_York

Launch Mattermost:

docker-compose up -d

Option B: Managed Deployment

[AFFILIATE_CALLOUT_HERE]

Setting up SSL certificates, configuring reverse proxies, and managing database backups adds complexity. If you want Mattermost deployed with automated SSL, monitoring, and daily backups already configured, managed platforms handle the infrastructure.

Step 3: DNS and SSL Configuration (20 minutes)

Set DNS A record:

chat.yourcompany.com → your-server-ip

Install SSL certificate (if not using managed hosting):

apt install -y certbot python3-certbot-nginx
certbot --nginx -d chat.yourcompany.com

Visit https://chat.yourcompany.com and complete initial setup:

Admin email: admin@yourcompany.com
Team name: Your Company
Create first team

Step 4: Install Slack Import Plugin (15 minutes)

Enable plugin uploads:

System Console → Plugins → Plugin Management
Enable "Upload Plugin"
Download Slack import tool: https://github.com/mattermost/mattermost-plugin-bulk-import
Upload via System Console → Plugins → Management

Configure Mattermost for import:

# Increase max file upload size
docker exec -it mattermost-docker_app_1 mattermost config set FileSettings.MaxFileSize 104857600

Saturday Morning: Data Migration (2-3 hours)

Step 5: Convert Slack Export to Mattermost Format (45 minutes)

Slack and Mattermost use different data structures. Use the official converter:

# Install mmetl (Mattermost Migration Tool)
wget https://github.com/mattermost/mmetl/releases/download/v1.0.0/mmetl
chmod +x mmetl

# Convert Slack export
./mmetl transform slack --team yourteam --file slack-export.zip --output mattermost-import.jsonl

What gets converted:

Channels → Mattermost channels
Users → Automatically created/matched
Messages → Preserves timestamps and threading
Reactions → Emoji reactions maintained
Files → Uploaded to Mattermost storage

What requires manual handling:

Custom Slack apps/bots (rebuild as webhooks)
Zapier integrations (use n8n or native webhooks)
Third-party apps (check Mattermost marketplace)

Step 6: Import Data into Mattermost (60 minutes)

Run bulk import:

# Copy import file to Mattermost container
docker cp mattermost-import.jsonl mattermost-docker_app_1:/tmp/

# Execute import
docker exec -it mattermost-docker_app_1 mattermost import bulk /tmp/mattermost-import.jsonl --apply --workers 4

Monitor import progress:

# Watch logs
docker logs -f mattermost-docker_app_1

Typical import times:

Small workspace (< 10K messages): 5-10 minutes
Medium workspace (10K-100K messages): 30-60 minutes
Large workspace (100K+ messages): 2-4 hours

Common import errors:

File size limits → Increase in System Console
Memory issues → Adjust Docker memory limits
User email conflicts → Pre-create user accounts

Step 7: Verify Data Integrity (30 minutes)

Check channel import:

# Compare channel counts
# Slack channels: Count from export
# Mattermost: System Console → Reporting → Site Statistics

Verify message history:

Open random channels in Mattermost
Search for known messages
Check file attachments load correctly
Verify @mentions and threading work

Test user accounts:

All Slack users should appear in Mattermost
Profile pictures imported (if available)
User statuses preserved

Saturday Afternoon: Integration Migration (2-3 hours)

Step 8: Recreate Critical Integrations

GitHub notifications:

# In Mattermost
/settings → Integrations → Incoming Webhooks → Add
Copy webhook URL

# In GitHub repository settings
Settings → Webhooks → Add webhook
Paste Mattermost webhook URL

CI/CD notifications: Replace Slack webhook URLs in your CI config:

# .github/workflows/deploy.yml
- name: Notify Mattermost
  uses: mattermost/action-mattermost-notify@master
  with:
    MATTERMOST_WEBHOOK_URL: ${{ secrets.MATTERMOST_WEBHOOK }}
    TEXT: "Deployment completed successfully"

Custom bots: Port Slack bot logic to Mattermost:

// Example: Simple notification bot
const axios = require("axios");

async function sendMattermost(message) {
  await axios.post("https://chat.yourcompany.com/hooks/xxx", {
    text: message,
    channel: "deployments",
  });
}

Step 9: Configure SSO (Optional, 45 minutes)

Connect to existing auth provider:

System Console → Authentication → SAML 2.0
Configure with Google Workspace, Okta, or Azure AD
Test with personal account before company-wide rollout

Benefits:

Single sign-on for all users
Automatic user provisioning
Centralized access control

Sunday: Team Preparation (1-2 hours)

Step 10: Create Migration Guide for Team

Send email 2 days before migration:

Subject: We're moving from Slack to Mattermost this Monday

Hi team,

We're migrating to Mattermost, our self-hosted communication platform. This saves us $8,400 annually while keeping all features you use daily.

What changes:
✓ New URL: https://chat.yourcompany.com
✓ Desktop app: Download from [link]
✓ Mobile apps: iOS/Android available
✓ All history preserved

What stays the same:
✓ Same channels and conversations
✓ Direct messages intact
✓ File sharing works identically
✓ @mentions and notifications

Action needed:
1. Download Mattermost desktop app [link]
2. Log in with your work email
3. Install mobile app (optional)

Monday morning checklist:
- Join team at 9 AM for quick onboarding
- Test notifications
- Verify you can access all channels

Questions? Reply to this email.

Step 11: Prepare Admin Team

Identify 3-5 "Mattermost champions":

Early adopters who can help others
Distribute across departments
Give admin training on Friday

Create quick reference guide:

Keyboard shortcuts comparison
How to set up notifications
Mobile app setup
Troubleshooting FAQs

Monday Morning: Cutover (2 hours)

Step 12: Final Sync and Launch (30 minutes)

Re-export Slack for weekend messages:

# Export Saturday-Sunday activity
# Run conversion again
./mmetl transform slack --team yourteam --file slack-export-weekend.zip --output weekend-import.jsonl

# Import delta
docker exec -it mattermost-docker_app_1 mattermost import bulk /tmp/weekend-import.jsonl --apply

Archive Slack workspace:

Slack Admin → Settings → Workspace Settings
Disable posting to all channels
Add workspace-wide message: "We've moved to Mattermost: [link]"
Keep read-only for 30 days

Step 13: Live Onboarding Session (60 minutes)

9 AM company call:

Welcome to Mattermost (5 min)
Quick tour of interface (10 min)
How to join channels (5 min)
Notification setup (10 min)
Mobile app demo (10 min)
Q&A (20 min)

First day support:

Monitor "help" channel
Respond to DMs quickly
Document common questions

Step 14: Decommission Slack (30 days later)

After 30-day grace period:

Export final Slack data for archive
Cancel Slack subscription
Download all Slack data
Store encrypted backup
Confirm cancellation

Savings realized:

Month 1: Pro-rated Slack savings + VPS cost
Months 2-12: Full $700/month savings
Year 2+: Compounding savings

Post-Migration Optimization

Performance Tuning

Database optimization:

# Enable search indexing
docker exec -it mattermost-docker_postgres_1 psql -U mmuser -d mattermost -c "CREATE INDEX idx_posts_message ON posts USING gin(to_tsvector('english', message));"

File storage optimization:

# Configure S3-compatible storage for files
# In System Console → File Storage
# Use Wasabi ($5.99/TB) or Backblaze B2 ($5/TB)

User Adoption Tactics

Week 1: Daily tips

Day 1: Keyboard shortcuts
Day 2: Advanced search
Day 3: Custom notifications
Day 4: Integrations
Day 5: Mobile app features

Week 2: Advanced features

Custom emoji and reactions
Channel organization
Saved posts and flags
Do Not Disturb schedules

Common Migration Pitfalls (And How to Avoid Them)

Pitfall 1: Incomplete data export

Solution: Export early, verify contents before migration weekend

Pitfall 2: Users forget new URL

Solution: Browser bookmarks, redirect old Slack URL

Pitfall 3: Integration breakage

Solution: Test all critical integrations in staging before cutover

Pitfall 4: Resistance to change

Solution: Involve team in decision, highlight cost savings

Pitfall 5: Lost custom emoji

Solution: Export custom emoji separately, bulk import to Mattermost

Migration Checklist

Friday:

[ ] Export Slack data
[ ] Deploy Mattermost server
[ ] Configure DNS and SSL
[ ] Install import plugin

Saturday:

[ ] Convert Slack export to Mattermost format
[ ] Import data to Mattermost
[ ] Verify data integrity
[ ] Migrate critical integrations

Sunday:

[ ] Send team migration guide
[ ] Train admin champions
[ ] Create quick reference documentation
[ ] Test all workflows

Monday:

[ ] Final data sync
[ ] Archive Slack workspace
[ ] Live onboarding session
[ ] Monitor and support users

The Exit-Saas Perspective

Migrating communication platforms isn't just cost savings. It's reclaiming ownership.

When you use Slack, Salesforce (owner) controls:

Price increases (15-25% annually)
Feature access (Enterprise features behind paywall)
Data retention (90-day limit on Free plan)
AI training on your conversations

When you self-host Mattermost:

Costs fixed at infrastructure level
All features available to everyone
Infinite message history
Complete data privacy

One weekend of migration effort saves your company $42,000 over 5 years. That's a senior engineer's salary.

Explore our tools directory for more self-hosted collaboration alternatives. The best time to migrate was when Slack doubled prices. The second best time is this weekend.