An Ireland debut tantalisingly eluded the 24-year-old wing in June when he was the only squad member not to taste Test action against Japan.
But McPhillips underlined his class when he spotted a gap in the Welsh side's rush defence after 53 minutes to run in the game's only try and set up Connacht's first home win in nearly eight months.
Cardiff, who trailed 10-3 after McPhillips' effort, kept battling and narrowed the gap to four points, but they had only three penalties from Nicky Robinson to show for their efforts at the finish.
An error-ridden opening period saw Connacht, who lost loose-head prop Ray Hogan to injury before the kick-off, struggle up front. Shoddy set-piece play allowed fly-half Robinson to kick Cardiff into a sixth-minute lead and the visitors, with nine internationals in their line-up, looked dangerous. However, Connacht got closest to making a breakthrough on the half-hour when David Slemen and Darren Yapp broke deep into Blues' territory only for the move to fall apart.
By that stage, Connacht had levelled. Following the 13th-minute sin-binning of the home flanker John Muldoon and the Wales lock Robert Sidoli for an off-the-ball scuffle, Paul Warwick slotted the resultant penalty.
After Robinson missed a chance to reply early in the second half, drilling a drop-goal attempt wide, McPhillips scorched through for his try.
Warwick did well to convert and the Australian held his nerve to kick a 74th-minute drop goal, sandwiched between a brace of Robinson penalties, to keep up Connacht's record of winning every one of their opening league games since 2001.