On 29 November 2018, Saka made his senior competitive debut for Arsenal in their Europa League match against Vorskla Poltava. He came on as a 68th minute substitute for Aaron Ramsey. On 13 December 2018, Saka made his first full home debut for Arsenal in their Europa League match against Qarabağ. On 1 January 2019, Saka made his debut in the Premier League in a 4–1 win against Fulham after coming on for Alex Iwobi in the 83rd minute. Saka scored his first senior goal on 19 September, curling a fine effort from long range into the corner away to German side Eintracht Frankfurt in the UEFA Europa League. He finished the match with two assists also to his name in a 3–0 win. Saka was rewarded for his efforts in Germany with his first Premier League start, in a 3–2 home win over Aston Villa. He then registered an assist for Pierre-Emerick Aubameyang, setting up the Gunners equaliser in a 1–1 draw at Old Trafford against Manchester United.

In May 2018, Saka was included in the England under-17 squad as they hosted the 2018 UEFA European Under-17 Championship. The hosts were eliminated by the Netherlands at the semi-final stage on a penalty shootout although Saka scored his spot kick.

In September 2018, Saka scored the winner for the England under-18 team in an away game against France. In November 2018, Saka received his first call-up to the England under-19 team. He scored on his debut at that age level in a qualifier against Moldova.

Following injuries to Sead Kolašinac and Kieran Tierney, Saka began to establish himself as a regular in the Arsenal first team at left-back. On 27 January 2020, he scored the opening goal in Arsenal’s 2–1 FA Cup fourth round victory at Bournemouth after finishing a 22-pass move, as well as assisting the second goal, scored by Eddie Nketiah. The goal was later voted goal of the round by the BBC. After a two-week mid-season break, Saka marked his return to action at home to Newcastle United with an assist for Nicolas Pépé’s goal. He then set up Alexandre Lacazette for his ninth assist of the season in a 1–0 win at Olympiacos in the UEFA Europa League. He then hit double figures for assists that season after crossing in for Nketiah in a thrilling 3–2 home win against Everton. Further performances against Portsmouth and West Ham United further strengthened his grip in the first team.

On 1 July 2020, Saka signed a new long-term contract with Arsenal. Head coach Mikel Arteta praised Saka and said: “I think he represents every value that this football club stands for. He has come through the academy, and earned his respect with hard work and accountability and you can see the progression that he is having as a player but as well as a person.” He scored his first Premier League goal for the club against Wolverhampton Wanderers, with a half volley beating goalkeeper Rui Patrício, in a 2–0 win. Saka was an unused substitute as Arsenal defeated Chelsea 2–1 to win their 14th FA Cup.

On 4 September 2020, Saka made his debut for England U21s during a 6–0 victory away to Kosovo.

On 1 October 2020, Saka was called up to the England senior squad for the first time, and made his international debut as one of the starters in a 3–0 victory over Wales.