Yes, email signatures are visible when users compose email in Outlook, OWA, and the OWA App for mobile devices.

However, email signatures are not visible when using the built-in email client on mobile devices, where signatures are injected by the forthcoming Symprex service for Office 365, or by the Email Signature Manager Transport Agent for On-Premises Exchange.

Alternatively, Email Signature Manager can be configured to email mobile device signatures to users as attachments that can be copied and pasted into the signature setting on the mobile device, so the signature is inserted when composing emails using the built-in email client.