The loss drops the conference-leading Lancers to 31-8 overall, 24-5 PacWest. The 'Lopes, who had won 10 of 13 coming in, improve to 20-17, 17-12.
The Lancers took a 3-1 lead with a pair of runs in the top of the eighth, but with one out in the eighth, Mendoza, the leading hitter in the PacWest, hitting .452 this year, connected on just his second homer of the year, ruining what was otherwise a dominant outing from starter Patrick Smith.
Smith (7-2) had scattered eight hits and allowed just one earned run while striking out five in 7.1 innings prior to giving up the game-winner on a 3-2 count.
The blast made a winner of Brady Wager (6-3), who gave up two earned runs and struck out six in eight innings. Mark Jarrett threw a perfect ninth for the save.
With the game tied 1-1 in the top of the eighth, Matt Daugherty hit a two-out, two-RBI single to give the Lancers a 3-1 lead, their first of the night. It was Daugherty's third hit of the night as he was 3 for 4.
The Lancers let a golden opportunity slip by in the top of the sixth when they put runners on the corners with no outs and the heart of the order coming up. But a failed squeeze attempt by Marnati and fly outs by Kevin Odom and Blair Moore ended the threat.
The 'Lopes took a 1-0 lead the next half inning on a leadoff single by Joey Bristyan, a sacrifice bunt and a double by Michael Pomeroy.
But the Lancers answered right back in the seventh, getting runners at first and second with two outs when pinch-hitter Michael Fagan reached on an error by second baseman P.J. Garcia that allowed Bullard to score the tying run from second base.
Bristyan, Pomeroy, Mendoza and Jackie Samora all had two hits for the 'Lopes.
The series continues tomorrow with a doubleheader (two 7-inning games) beginning at 3 p.m.