Mashimbyi charges fell just short of the 277-run target, which would have been a record run-chase in 50-over internationals for the Proteas, with opener Tazmin Brits scoring 109.
The Proteas collapsed from 240-4 to eventually be bowled out for 261 in the final over and Mashimbyi rued his team's missed opportunity to start the series with a victory.
The head coach said: "I'm really proud of the girls. There are a lot of positives we can take out of this game. I think they showed a good fight throughout.
"Obviously it didn't go our way, which was quite unfortunate. But I think for me, as I summarised the whole game, it was a game of missed opportunities.
"For the majority of the match, we actually had control of the game, and we let it go away from us and that's when they capitalised with both bat and ball."
The tri-series in Sri Lanka is Mashimbyi's first full series in charge of the national side and he has identified areas which the team must improve before taking on the hosts on Friday.
He added: "There are so many things that we can take out. In the field, we were a bit sloppy, so that’s something we can work on.
"The nice thing about this game for me is to identify the areas that we can really improve and grow in. That's why we're never here to lose, but we're here to learn and grow."
The Proteas Women completed a clean sweep in the five-match T20I series against Zimbabwe in Bulawayo, although the hosts put up a much improved showing to only go down by 13 runs in the final contest on Saturday.
South Africa Women produced a dominant display against Zimbabwe Women, securing an 80-run victory in the fourth match of their tour of Zimbabwe.
Laura Wolvaardt struck a career-best 126 not out as the Proteas Women beat Zimbabwe by 80 runs in the third T20I in Bulawayo as the visitors wrapped up the series with two matches to spare.
The Proteas Women went 2-0 up in the five-match T20I series against Zimbabwe courtesy of a 64-run win in Bulawayo on Sunday.
