Former Lions striker Gary Alexander was on target with a second-half penalty to seal victory for Crawley as Millwall's pre-season continued with a tough work-out at Crawley Town.
Billy Clarke had earlier put The Reds ahead against the run of play on the half-hour mark before Alexander wrapped up victory from the spot just four minutes into the second-half.
Although the two teams will meet again in the Capital One Cup in just over two weeks time, this proved to be a useful exercise for The Lions.
Boss Kenny Jackett was able to hand both Darius Henderson and Liam Trotter their first match action of the pre-season, and in Henderson's case, his first taste of senior football in around five months after missing the last seven games of the 2011/12 campaign through injury.
The Lions certainly created a stack of chances, and it was a fair reflection on the game that Crawley keeper Paul Jones was named as Man of the Match for his athletic display between the sticks.
Millwall waited 12 minutes to test Jones, and then did so twice in as many minutes.
Andy Keogh's attempt to steer the ball past the keeper from a lovely Josh Wright pass was touched round the post by Jones. Wright then went close himself with a fierce drive that was parried by Jones at full stretch.
Wright tried his luck again on the quarter hour with a free-kick on the right hand side of the penalty area, but on this occasion his curling effort drifted wide of the upright.
At the other end Crawley's Dannie Bulman sent a 25-yard fizzer a yard wide following as the home side threatened.
Lions midfielder Wright nearly had another 'Burnley' moment with a rasping 20th-minute volley that was inadvertently cleared by the head of Nicky Adams, who took the full force of his effort.
However play broke quickly up the other end and a long ball picked out Clarke on the left, who took a couple of strides forward before slamming an angled drive beyond the reach of Maik Taylor into the bottom right corner to piut Crawley 1-0 up.
The Lions could have levelled immediately but Keogh's shot was brilliantly blocked by Jones, who then got the slightest of touches to divert Henderson's bobbling goalbound effort inches wide with the last action of the half.
There were no changes to either side as the second-half resumed - but there was a change to the scoreline within four minutes of the restart. Karleigh Osborne's attempted pass to Alan Dunne was intercepted by Clarke, who raced forward. However his charge into the penalty area was brought to an abrupt halt when he was sent tumbling by Osborne and Alexander grabbed the ball to calmly tuck home Crawley's second from the spot, sending Taylor the wrong way.
Lions boss Jackett replaced Henderson and Trotter on the hour mark and the game thereafter was punctuated by a plethora of substitutions - eight for the home team and a further one by Millwall.
The visitors lost their way briefly for a spell in the middle of the half, but recovered sufficiently to finish the game strongly.
Barron and Keogh were both thwarted by fine saves from keeper Jones, although Crawley sub John Akinde fired woefully off target from 10 yards with the goal gaping.
First blood may belong to Crawley, but with a further two pre-season games and fine-tuning by Millwall before that capital One Cup clash, it promises to be a fascinating encounter at The Den on 14th August.
Crawley Town: Jones, Connolly (Hunt, 62mins), Davis (Wassmer, 74mins), McFadzean, Sadler (John, 88mins), Akpan (Torres, 68mins), Simpson (Davies, 68mins), Bulman (Cooper, 68mins), Adams (Nielsen, 68mins), Alexander, Clarke (Akinsde, 74mins). Subs not used: Kuipers, Wickham, Smith, Evans.
Scorers: Clarke (30mins), Alexander (49mins, pen)
Millwall: Taylor, Dunne, Malone, Wright, Smith, Osborne, Racon, Trotter (Batt, 60mins), Taylor (Barron, 65mins), Keogh, Henderson (Feeney, 60mins). Subs not used: Forde, Marquis, Ward, Goodman, N'Guessan.
Ref: T. Robinson