The Senators first season in the NHL, 1917 – 1918, did not go well. Salary squabbles delayed the home opener (on the league’s first night, December 19, 1917) as players protested that their contracts were for twenty games, while the season schedule was for twenty four. Enough players were appeased that the game started, 15 minutes late, while two players Hamby Shore and Jack Darragh, stayed in the dressing room while negotiations went on. The Senators lost their home opener 7 – 4. The Senators lost their previous top rival, the Wanderers, after five games. The team struggled and finished in third place after the first half of the season. The club made player changes in the second half, getting Horace Merrill out of retirement and releasing Dave Ritchie. It was Shore’s last season as he would die of pneumonia in October 1918. Shore’s last career game was in the third-last game of the season and he was sat out for the last two games. In the end, the team placed second in the second half and missed the playoffs. Cy Denneny led the team, coming second overall in scoring in the league with 36 goals in 20 games.

The 1920 Stanley Cup Finals was contested by the National Hockey League (NHL) champion Ottawa Senators and the Pacific Coast Hockey Association (PCHA) champion Seattle Metropolitans. The Senators won the series by three games to two in the best-of-five game series. Although all of the games for the series were scheduled to be played at The Arena in Ottawa, unseasonably warm weather and poor ice conditions forced the last two contests to be played on the artificial ice at Toronto’s Arena Gardens.

This would be the last Stanley Cup Final appearance for a team based on the West Coast of the United States until the 1993 Final.

The 1921 Stanley Cup Finals was contested by the National Hockey League (NHL) champion Ottawa Senators and the Pacific Coast Hockey Association (PCHA) champion Vancouver Millionaires. The Senators defeated Vancouver three games to two in the best-of-five game series to become the first team to win back-to-back Cup championships since the 1912 – 1913 Quebec Bulldogs.

The 1923 Stanley Cup Finals was contested by the NHL champion Ottawa Senators and the WCHL champion Edmonton Eskimos. The previous WCHL-PCHA playoff format was abandoned, and the Ottawa Senators now had to play first the PCHA champion Vancouver Maroons, followed by the WCHL champion Edmonton Eskimos in the Final.

This would be the last Final until the 1983 Final to be contested by a team from Edmonton. Both games were played in Vancouver, making this the last Final played entirely at a neutral site (the 1924 and 1925 Finals each featured one neutral site game).

The 1927 Stanley Cup Finals was played by the Ottawa Senators and the Boston Bruins. It was the first time the Cup was solely contested by National Hockey League teams, owing to the demise of the Western Hockey League the previous year. It was won by the Ottawa Senators, coached by Dave Gill, over the Boston Bruins, coached by Art Ross. This was the Senators’ fourth win since 1920, and eleventh overall, but it marked the end of the dynasty. The original Senators would not win another.

It would be the last time a team from Ottawa would play in the Finals until the 2007 Stanley Cup Finals, and the first Stanley Cup Final to have games played in both Canada and the United States.

Despite finishing in last place for the second year in a row, the Senators actually improved their attendance over the previous season. Even with the increased gate, they barely survived the season. After the season ended, it was announced by Auditorium president F. D. Burpee that the franchise would not return to Ottawa for the 1934 – 1935 season due to losses of $60,000 over the previous two seasons. The losses were too great to be made up by the sale of players’ contracts, and the club needed to be moved to “some very large city which has a large rink, if we are to protect the Auditorium shareholders and pay off our debts.” The NHL franchise was moved to St. Louis, Missouri and operated as the St. Louis Eagles. The Eagles played only one season, finishing last again. They suspended operations after the season, never to return. Flash Hollett was the last member of the Senators to play in the NHL, retiring with the Detroit Red Wings in 1946.

The city of Ottawa did not have an NHL franchise again until the new Ottawa Senators franchise was awarded for the 1992 – 1993 season. The NHL presented the Senators with a “certificate of reinstatement” commemorating Ottawa’s return to the league, and the current Senators honor the original franchise’s 11 Stanley Cups. However, records for the two teams are kept separately. Frank Finnigan, the last surviving member of the original Senators’ last Stanley Cup winner, played a key role in the drive to win an expansion franchise for Ottawa. He was slated to drop the puck in a ceremonial face-off for the new franchise’s first game, but died a year before that game took place. The new Senators honored Finnigan by retiring his #8 jersey.