It turns out there's a whole lot of data out there in the world, some of which doesn't pop up with a simple Google search :)
Peter Metropulos and Chris Heyward informed me of an Am Redstart breeding record from the 1997 San Mateo Co breeding bird atlas. Fledged young were 0 AMRE and I believe multiple Brown-headed Cowbirds. I wanted further details so I searched literature Peter and Chris referenced. In the National Aubdubon Society Field Notes Vol. 51, Issue 5 (winter) from 1997, it is reported that breeding evidence of American Redstart is easily obtainable in Humboldt and Del Norte counties. It then goes on to mention that that '97 summer had the first proven records of AMRE breeding in Marin, San Mateo, and Monterey counties. These were records obtained by the likes of Ron Thorn, Rich Stallcup, and Don Roberson.
Perhaps on the GGAS Chat forum (Dominik, is that the preferred discussion place?), other California field veterans could share any other pertinent Redstart info of the last 50 years.
All that said, I am 99.9% certain the Mtn Lake record is a first for SF county.