This is expected behavior. macOS and iOS do not sync Contacts directly between devices anymore unless you use iCloud or another online account.
What’s happening:
• Contacts added locally on the Mac stay on the Mac
• Contacts added locally on the iPhone stay on the iPhone
• When you enable iCloud Contacts, Apple uses it as the sync service, so changes appear on both devices
Apple removed direct Mac ↔ iPhone contact syncing years ago. Finder/iTunes syncing no longer handles Contacts — only backups, media, etc.
Important note:
Even if both devices are signed in with the same Apple ID, Contacts will NOT sync unless a sync service (iCloud, Google, Exchange, etc.) is enabled.
Options:
Use iCloud Contacts (recommended)
Mac: System Settings > Apple ID > iCloud > Contacts
iPhone: Settings > Apple ID > iCloud > Contacts
Use another account (e.g. Google, Exchange, CardDAV) on both devices
Manually export/import contacts (not automatic)
Nothing is broken — this is how modern macOS and iOS are designed.
Official Apple support documents:
• Apple: Set up iCloud Contacts
Continuity Camera: Use iPhone as a webcam for Mac - Apple Support
• Apple: Add or remove accounts on iPhone
Set up Exchange ActiveSync on your iPhone, iPad, or Apple Vision Pro - Apple Support
• Apple: Use Contacts on Mac
Contacts User Guide for Mac - Apple Support