The midfielder, who left Adams Park last June, smashed in the winner two minutes after the restart and that proved enough to down the high-flying Chairboys.
Left-back Scott Laird was sent off for two bookable offences but Stevenage held out for a deserved win.
Boro were ahead two minutes after the restart when Mousinho let fly from 25 yards to beat Nikki Bull after Rob Sinclair's rampaging run forward.
Laird picked up his second yellow for a crude foul on Scott Rendell but despite the man advantage Wycombe failed to test Chris Day in the closing stages.
Source: PA
Source: PA