On 2 August Dumbarton defeated St Mirren 2–0 in the semi-final, but lost out a week later to the hosts, Rangers, 4–1 in the final.

On 16 August the opening games in the inaugural Scottish Football League championship were played, and Dumbarton began their campaign with a home fixture against Cowlairs.

The game was held in good conditions apart from a strong wind blowing down the pitch meaning most of the play was confined to one end.

Hearts were first to score but Bell with his third goal in three league games equalised matters just before half time.

The Sons came out in the second half intent on pushing forward and goals from Bell and Mair secured a 3–1 win.

Dumbarton were drawn away against Smithston Hibs in the first round of the Scottish Cup on 6 September, but as the opponents did not have a suitable standard of ground the match was switched to Boghead.

Alex Miller took McDonald's place in the half back line, The Sons were firm favorites for this tie and were a goal ahead from McNaught within five minutes.

Dumbarton continued to play strongly in the second half and defeated their rivals 5–1, Bell completing his hat-trick.

After a free week, it was back to Scottish Cup business on 27 September, with Hibernian being the opponents at Easter Road.

The Dumbarton men spend most of the first half swarmed around the Cowlairs goal with scores resulting from three scrimmage and a Bell strike giving the Sons a 4–0 lead.

Following a free week it was back to Scottish Cup business on 8 November, with a fourth round tie against Mossend Swifts at Boghead.

Mossend then responded with two goals of their own before McMillan, Mair and Taylor gave Dumbarton a 5-3 half time lead.

On 15 November, eight of the Dumbarton first XI - McLeod; Watson; McMillan; Boyle; Taylor; Galbraith; Mair and McNaught – were selected to play in the county side against Renfrewshire.

Bell Galbraith and Taylor had the Sons well ahead early in the first half, and at the interval the score stood in Dumbarton's favour by 4–2.

On 29 November it was to have been the fifth round home tie in the Scottish Cup against 5th KRV, but the referee ruled the pitch unplayable.

An unchanged side took to the field The Sons had little difficulty in breaching the KRV defence and were 6-0 up by half time - Taylor notching a hat-trick (his fourth of the season so far).

On 20 December, the biggest match of the season so far saw 10,000 spectators crammed into Boghead to witness Dumbarton play Celtic in the Scottish Cup quarter final.

So at the end of 1890, Dumbarton remained unbeaten – leading the league with 15 points from 8 games, six ahead of Rangers - and still going strong in both the national and county cups.

The 1891 Scottish Cup Final was played on 7 February at Hampden Park, pitting the odds on favorites unbeaten Dumbarton against the surprise package Hearts.

Celtic came back in the second half and equalised, and although Boyle had the home team ahead once more, the Celts had the final say the game ending a 2–2 draw.

Wilson took the place of McNaught in the attack It only took two minutes for Keir to put Dumbarton ahead, and Taylor and an own goal made it three by half time.

The Sons were to find the visitors' keeper in fine form and it was Cambuslang that would start the second half a goal ahead.

On 28 March Third Lanark arrived at Boghead for the return league fixture, where an unchanged side took the field.

The Dumbarton men were anxious to make up in some way for the defeat in the Scottish Cup final and were one ahead from Bell at half time.

The Gers scored again in the second half before two quick fire goals from Bell brought the Sons back into the game.

The Saints recovered to score midway through the second half, though J Miller restored the three goal lead shortly thereafter.

On 3 June the final of the Greenock Charity Cup was decided at Cappielow as Dumbarton played hosts Morton for the second year in succession.

It was more of the same in the second half and two goals from Taylor ended the season on a high with a 3–0 win and yet another piece of silverware for the Dumbarton trophy cabinet.

[7][6] Amongst those leaving the club from the first XI were Ralph Aitken to Newcastle West End, Duncan Stewart to Port Glasgow Athletic and Joe Lindsay to Renton.

The first XI was enhanced by John Taylor and Daniel Watson stepping up from the reserves and also James McNaught joining from Methlan Park.