Winger Imhoff slid in to score in the 20th minute and although Italy's Michele Campagnaro crossed for a try in his first international start, Tommaso Allan's four missed penalties proved costly for the hosts.
Jacques Brunel's side Italian side were crushed by Australia before edging to a 37-31 win over Fiji last week, a victory they almost threw away despite their opponents being down to 11 players at one point.
After two Allan penalties the away side scored the opening try through Imhoff who finished off a direct attack which exposed flaws in Italy's defending and made the score 7-6.
Italy struggled to break down Argentina's resilient defence and had to rely on Allan to put them back in front with a penalty five minutes later.
Sanchez kicked a penalty to put Argentina 10-9 ahead at halftime and slotted another to make it 13-9 before Campagnaro nipped in to snatch the lead back for the hosts.
In a match dominated by the boot, however, it was fitting that Sanchez sealed his side's win, kicking a 67th-minute penalty before slotting over a classy drop-goal with eight minutes left.
(Reporting by Terry Daley, editing by Ed Osmond)
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