It depends on when your action takes place. Are you speaking about something that happened today, yesterday in the past? Because of the use of as, you must use the two verbs in the same tense, as it implies simultaneity.
Consider the following options
- if the actions occurred in the past
As I mingled with the crowd, I kept an eye out for my friend.
As I was mingling with the crowd, I was keeping an eye out for my friend.
- if the actions are taking place in the present
As I am mingling with the crowd, I am keeping an eye out for my friend.
I am not saying that I have exhausted all the possibilities, but given the vagueness of the situation in time in your context, this is what I came up with.