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."
Annerie Dercksen scored her maiden international century while Chloe Tryon starred with bat and ball to help the Proteas Women power to a 76-run victory against Sri Lanka in the ODI tri-series in Colombo.
Annerie Dercksen and Chloe Tryon starred with the bat but it was not enough as the Proteas Women slipped to a 23-run defeat against India in the ODI tri-series in Colombo.
Head coach Mandla Mashimbyi admits the ODI tri-series in Sri Lanka is a "learning curve" for the Proteas Women as they look ahead to the World Cup later this year.
The Proteas Women fell to their second defeat in as many games in the ODI tri-series in Colombo after a five-wicket loss against hosts Sri Lanka on Friday.
