In the depths of the depression, a small time con artist teams up with a seasoned pro to con $500,000 from a ruthless mobster to avenge the mobster’s murder of his mentor.

This story has a lot of possibilities. It could be a comedy like "The Sting" or a thriller like "The Godfather." A lot will depend on the character of the con artist.

I wasn’t sure who the mentor had the relationship with but it seems to have been clarified. It looks like something I’d enjoy seeing

The pro is seasoned but in a slump. And the man killed by the mobster was a beloved friend of the pro.

Nir:

I believe it is more effective for the logline to specifically indicate the method for getting revenge, to wit, pull off a long con. As if there was another option given the characters' profession. But still.

A great example, I'd make the wording a bit more "active sounding".
For example:
After a ruthless mobster kills his mentor, a small-time con artist takes revenge by teaming up with a seasoned pro to get half a million out of the murderer.

Only thing I am not sure of is what is the inciting incident is (Death of partner, offer to rob)

Like Karel mentioned, I think putting the death up top provides motivation

Also why does the pro want in? If a pro I imagine doing well. If the pro was ‘washed up’ or this was a chance at redemption as well?