In sweltering conditions at New Dundas Park Rose saw off a fightback by the visitors to earn a hard-fought 2-1 victory.
Bonnyrigg took the lead in 25 minutes when Lee Currie, who had earlier struck the post, found the net with a superb half-volley.
George Hunter’s deflected strike double the Rose advantage early in the second period. But former Watford defender Bosun Lawal halved the deficit just before the hour and Rose had to dig deep to see the game out.
Horn said: “I was delighted to get the three points. It was a really tough game in tough conditions with the heat and playing against a full-time team.
"I thought we started the game really well and probably should have been further in front than we actually were.
"We eventually got the second goal in the second half and then lost a goal and we had to dig deep. They put us under a bit of pressure but we defended pretty well to be fair and managed to see the game out.”
After the off-the-field controversy surrounding the inclusion of Old Firm colts’ sides in the Lowland League, the match was a first chance to assess how they will fare on the pitch – and Horn admitted he was happy to have first crack at the Celtic youngsters.
He said: “I think Celtic will improve as the season goes on.
"They were missing a few players and there were quite a lot of their players involved with the first team on Saturday as well so it was probably a good time to play against them."
"It was tough. We had water breaks which is not the norm for Scottish football.
"It was a big test of character for us but we stood firm and managed to get ourselves over the line."
After Wednesday night’s match at early leaders Cumbernauld Colts, Rose are away again this Saturday, to Civil Service Strollers.
Horn said: “It will be a tough game. In the last couple of seasons we've had difficult games against Civil and I don't expect anything less on Saturday.
"They will be well organised and up for the battle and Bonnyrigg are seen as a scalp in the lower leagues. Everybody wants to beat us.”