The Frenchman, fit again after a lengthy lay-off following thigh surgery, pounced from close range as City interrupted their Champions League quarter-final double-header against Paris Saint-Germain to claim three crucial Barclays Premier League points.
Stephane Sessegnon had fired the Baggies into a sixth-minute lead ith a stunning strike at the Etihad Stadium but the same player then conceded the penalty that allowed Sergio Aguero to equalise.
Aguero limped through the latter stages after a challenge from Craig Gardner but remained a willing runner until being taken off in the 89th minute.
His fitness could be a concern for Tuesday’s visit from PSG, which also influenced selection to the extent of five changes to the starting line-up.
Side issues saw fans continue to protest about the price of tickets for Tuesday and Wilfried Bony again struggle up front, but City tightened their grip on fourth place and Nasri finally had something to cheer after a horrendous season.
City were caught out early as the visitors broke from deep to snatch the lead. Fernando – as in Paris in midweek – lost possession and Nasri could not recover as referee Michael Jones blocked his path.