The Scots went into the match as favourites in Queenstown following a warm-up campaign which comprised five straight wins, only for a poor display in the field to allow Hong Kong to post 263 for seven.
Scotland's batsmen then fell a full 17 runs short despite Mommsen's heroics. "Our performance was simply not good enough," said Kyle Coetzer, the Scotland captain. "We let standards fall well short of what we set ourselves in each discipline but we have to put it behind us and move forward."
That is the prerogative of a beaten captain but Scotland's performance is worth recounting here. After winning the toss and electing to field, Coetzer was afforded the perfect start when Mark Wright was run out with just five runs on the board. Irfan Ahmed would lead a recovery, though, with a battling knock of 75, including two boundaries and three 6s.
Scotland had the chance to reclaim the initiative when Ahmed became one of Iain Wardlaw's three victims but Hong Kong were able to add 104 runs in the last 12 overs as the Scottish bowlers toiled. Leicestershire's Rob Taylor - who had claimed four wickets in each of the previous two warm-up games - came in for the heaviest punishment, conceding 65 runs during his nine overs.
Coetzer's side fared no better with the bat, with the captain falling for seven to set the tone for a faltering run-chase. Only Mommsen showed the required sense of purpose with a score of 118 runs from only 109 balls.
The Carlton batsman - making his first competitive appearance for five months following pelvic surgery - struck 13 boundaries and one 6. However, he was given no sustained support and was eventually last out as his one-man rescue mission ended in disappointment.
"It's a shame nobody was able to stay with him long enough to get us over the line," added Coetzer. "It's not an ideal start but our aim will simply be to play better in the next game and take it from there."
That comes in the form of a crucial match with Nepal on Thursday. The Scots must finish among the top three of their qualifying group in order to progress to the Super Six stage and stay in with a chance of claiming one of two places on offer at the 2015 World Cup.