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."
Nadine de Klerk once again proved to be the hero with the bat as the Proteas Women held their nerve to beat Bangladesh by three wickets with three balls to spare at the ICC Women's World Cup.
Nonkululeko Mlaba took four wickets before Tazmin Brits struck a century as the Proteas Women bounced back at the ICC Women's World Cup with a six-wicket victory against New Zealand in Indore.
South Africa Women made a powerful statement ahead of the ICC Women's Cricket World Cup as they claimed an eight-wicket victory over Pakistan in the first ODI at the Gaddafi Stadium in Lahore on Tuesday.
The Pakistan Women earned a consolation win when they defeated the Proteas Women by six wickets on a scorching day in Lahore on Monday.
