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."
Proteas Women captain Laura Wolvaardt has expressed her excitement at the opportunity to face a fellow African opponent for the first time when they take on Zimbabwe in a five-match T20I series in Bulawayo starting on Friday.
Laura Wolvaardt will lead a youthful Proteas Women squad, including three uncapped players, for next month's historic five-match T20I series in Zimbabwe.
Marizanne Kapp is the only Proteas player selected in the ICC Women's T20 World Cup team of the tournament, a trophy won by Australia when they beat England in the final at Lord's on Sunday.
The Proteas Women fell to a 40-run defeat against host nation England in the ICC Women's T20 World Cup semifinal at the the Oval on Thursday.
