M1 can make transferring another brokerage account to an M1 account easy.
You'll first want to make sure you have an account open with M1 that matches the account type being transferred. For example, if you are transferring a joint brokerage account, you will want to open a joint account with M1.
Then, to initiate a transfer, please email firstname.lastname@example.org. In the email, include a copy of your most recent monthly statement from the brokerage account you'd like to transfer to M1 and any specific instructions for the transfer. After you send the email, our transfers team will handle the rest.
Attached to this article is a PDF that describes the account transfer process in detail.
If you have any specific questions, never hesitate to email the team at email@example.com.