Seydou Doumbia gave Roma the lead 10 minutes before halftime, scoring for the second game in a row after the ball broke kindly for him in the Genoa area.
The Ivorian joined Roma from CSKA Moscow in January and endured some harsh criticism before breaking his duck in the 3-0 win at Sassuolo in midweek.
Sixth-placed Genoa, themselves chasing a place in the Europa League, produced plenty of chances in the second half and were pressing for an equaliser when Roma struck again in stoppage time.
Florenzi dispossessed Tino Costa midway inside the Roma half, burst clear with Costa in chase and ended his 60-metre sprint by rifling an unstoppable shot into the roof of the net from just inside the area.
Roma are second with 64 points from 33 games, two ahead of Lazio who were away to Atalanta later on Sunday (1300 GMT).
Second place earns a spot in next year's Champions League group stage while third earns a place in the playoff round. Juventus wrapped up the title with a 1-0 win at Sampdoria on Saturday. (Reporting by Brian Homewood, editing by Alan Baldwin)