England had the better of the first-half but fell behind to Flavia Severin's try, before hitting back through Alex Matthews and Abbie Scott to lead at the break.
Ceri Large extended the lead before Mclean, Hannah Gallagher and Kay Wilson crossed late on as Nicky Ponsford's side condemned Italy to their second consecutive defeat following their opening 30-5 reversal against Ireland.
England enjoyed the better of the opening exchanges and were quickly in front, returning fly-half Mclean kicking an easy penalty after four minutes.
Mclean should have doubled the lead soon after but dragged another penalty wide as the hosts enjoyed the majority of the early territory and possession.
However it was the Azzurri who claimed the game's first try, several rumbles from the forwards ultimately seeing lock Severin cross, Michela Sillari converting.
But not long later flanker Matthews scrambled over from close range after England patiently went through the phases, Mclean's conversion making it 10-7.
Mclean ambitiously went for goal from 45 metres but her kick squirted low and well short but soon England grabbed their second try, lock Scott crossing from close range on her full international debut. Mclean pushed the conversion to leave it 15-7 at the break.
England's forwards looked to apply the squeeze on the resumption, pressure at the scrum earning multiple penalties before Italy's fly-half Beatrice Rigoni was sin-binned for offside.
The hosts were guilty of making errors at crucial moments, Scott suffering from white-line fever and getting held up and bad handling errors meaning they were unable to extend their lead despite controlling possession and territory again.
Eventually the third try did come, patiently retaining the ball and feeding it right where centre Large burrowed over from close range, Mclean converting.
And Mclean sealed the win when she took a quick tap penalty and wrestled the ball down from five metres out, though she could not convert her own score.
Openside Gallagher then picked a gorgeous line and showed impressive pace to race in from range, replacement Megan Goddard converting from out wide.
And in the final play of the game full-back Wilson crossed for an unconverted score to see the hosts end on a high ahead of their trip to face unbeaten Ireland Women in a fortnight.
– but the sailors praise the new boats