The 19-year-old, having this week extended his loan stay from Watford, took just 40 seconds to grab his first professional goal on what was his first-ever league start, driving in from close range after Oli Johnson's initial effort had been blocked following a swift break.
Colchester's Kayode Odejayi struck a post with a header from close range before the U's deservedly got back on level terms seven minutes before the break.
After good play by Ian Henderson down the left, Andy Bond's long-range drive deflected off a Yeovil defender before looping past helpless keeper Jed Steer. And Colchester were ahead nine minutes after half-time when Michail Antonio headed in Ben Coker's deep cross at the far post.
But Massey struck again just after the hour mark when he followed up to score from close range, with Colchester keeper Ben Williams having only parried Max Ehmer's low drive.
Glovers substitute Kieran Agard saw his close-range header rattle the bar in stoppage time, as an entertaining contest finished all square.