The 18-year-old was the only player to feature in both halves of a 2-1 victory over Charlton in Newport.
Montgomery made an impressive first-team debut in a 4-0 win against St Johnstone in May and is aiming to be involved when the season kicks off for real.
And his versatility could be a major asset, with Celtic short of options on the left side following the departures of loan players Diego Laxalt and Mohammed Elyounoussi.
"First half I enjoyed playing left wing, a bit further forward, and then second half back to left-back, so it was a good run out," Montgomery told Celtic TV.
"I enjoy both. I would say my preference is probably left-back now after playing it a bit more, but I can do either or.
"I was a left-winger since growing up so I have still got that in my game and I can still play that position but, with the way we are starting to play now, I think left-backs will be attacking much more, so I can bring that to my game from further back on the pitch.
"Last season one of my goals was to try to get a debut and break in before the end of the season. This season I will continue to work hard and see where that takes me."
Another teenager to impress was Karamoko Dembele, who netted along with Albian Ajeti.
Summer signings Liam Shaw and Osaze Urhoghide both featured along with Olivier Ntcham, who had appeared to be on his way out of Celtic Park this summer.
One notable absentee was Kristoffer Ajer, who continues to be the subject of transfer speculation.
A report in Norway claimed Celtic had rejected a £12million offer from Brentford which could have been worth a further £3m and that the 23-year-old defender now looked set to see out his contract next season.