The former All Blacks legend had a clear run-in for the long-awaited touchdown in the opening minute of the second half.
Cardiff went on to finish with six tries and a vital bonus point.
Fly-half Nick Robinson claimed 16 points in his 100th match for club and region, as the Blues completed a Christmas double, having beaten their other regional rivals the Ospreys last week.
The Dragons made the best possible start silencing the home crowd with a Nathan Brew try after only 21 seconds of play.
But after that it was one-way traffic as the Blues took control.
Robinson kicked two penalty goals and converted his own try after a brilliant opportunist effort.
Craig Morgan took full advantage of a huge overlap to claim the Blues’ second try and on the brink of half-time Ryan Powell added another.
The Dragons were living off scraps of possession and relied on two Ceri Sweeney penalty goals for their points.
Lomu’s try sparked an action-packed second half.
Hooker Rhys Thomas’s try for the Blues was cancelled out by one from his namesake for the visitors.
Jon Bryant added another for the Dragons before Rhys Williams completed the scoring with the Blues’ sixth try 20 minutes from the end.
The win sees the Blues climb two places to sixth in the table.
Cardiff Blues scorers: N. Robinson try, 3 pens, con, Morgan, R. Powell, J Lomu, R. Thomas, R. Williams try each.
CARDIFF BLUES: Jenkins, R. Thomas, M. Jones, Quinnell, Sidoli, A. Powell, Sowden-Taylor, Rush, R. Powell, N. Robinson, Lomu, Davies, J. Robinson, Morgan, R. Williams. Replacements: Goodfield for R. Thomas (71), M. Williams for A. Powell (68), Phillips for R. Powell (58), Warren for N. Robinson (71).
Newport-Gwent Dragons scorers: Brew, R. Thomas, Bryant try each, Sweeney 2 pens, con.
NEWPORT-GWENT DRAGONS: Maddocks, Brown, Robinson, Charteris, Sidoli, Hall, Davies, Owen, Cooper, Sweeney, Brew, Bryant, Luscombe, Chapman, Morgan.
Replacements: Black for Maddocks (50), R. Thomas for Robinson (50), Griffiths for Charteris (50), Oakley for Hall (73), Fussell for Brew (39), Warlow for Morgan (22).
Meanwhile, Barry Everitt salvaged a dramatic 20-19 victory for London Irish with the last kick of a hot-tempered Guinness Premiership game at Vicarage Road yesterday.
The fly-half fittingly drilled over a 40-metre penalty to finish with 15 points, having also supplied the creative spark for Michael Horak’s first-half try.
After the interval, tempers boiled over in a furious 25-man scrap sparked by a clash between Ben Skirving and Delon Armitage that saw both sin-binned.
Elsewhere, Matt Burke’s flawless kicking display proved the difference as Newcastle snatched a vital 13-10 away win against Leeds in a tight relegation battle at Headingley.
After the day’s other premiership game, Leicester coach Pat Howard had to take stock of his team’s miserable performance at newly-promoted Bristol.
Leicester, who have serious Premiership and Heineken Cup ambitions this season, were kicked to defeat by a Jason Strange-inspired Bristol at the Memorial Stadium.
Strange booted five first-half penalties for a 15-3 triumph, but Leicester never seriously threatened to challenge their hosts and spent most of the match on the back foot.