Joe Johnson (two), Charlie Osborne and Dan Sibley got the goals as USP recorded a 4-3 triumph on the road.
Player-assistant manager Jeffes felt it was a three points his troops fully deserved for their efforts.
And Jeffes says it was another step in the right direction with US Portsmouth sitting with 11 points on the board from seven league games after winning promotion to the Wessex Premier last season.
Jeffes said: ‘I’m really, really pleased after yesterday. We played Portland a few weeks ago so we sort of knew what to expect - it’s never an easy place to go.
‘It took everyone three hours to get there and I thought we applied ourselves really well at what is a tough place to go.
‘They’re quite direct in the way they play and they’ve got some experienced players. I felt across the board everyone did themselves justice and put a real shift in.
‘To have 11 points after seven games and be where we are in the league is really pleasing.
‘We’re under no illusions that we’re still building, it’s all new, there’s only two of us left who started the Vase game (FA Vase semi-final last season) so it’s really, really pleasing - without getting carried away.’
US Portsmouth did not appear fazed by the long trip down to Dorset as they took the lead inside a minute through Osborne.
Back came Portland, levelling through Lewis Clarke on 23 minutes, but Johnson restored the visitors' advantage five minutes later.
The hosts drew level for a second time 11 minutes before the break but Johnson put US Portsmouth back in from after 41 minutes.
An eventful opening 45 minutes, which saw six goals, was ended with Sibley striking a minute before the half time whistle.
Sam Poole set up a nervy final few minutes for the visitors, netting three minutes from time, but they saw it out to record a third league win of the season.