Mahmood held talks with US Homeland Security Secretary Kristi Noem, who oversees the US Immigration and Customs Enforcement (ICE) agency and has been a key player in the Trump administration's efforts to ramp up deportations.
The Five Eyes alliance is a decades-old intelligence-sharing pact between the US, UK, Canada, Australia and New Zealand. It is often described as being among the most successful agreements between allies for sharing classified intelligence.
Online child sexual abuse and the spread of opioids were also on the agenda at the summit, which was attended by Canadian public safety minister Gary Anandasangaree, Australian home affairs minister Tony Burke and New Zealand minister Judith Collins.
Earlier on Monday, Mahmood said she hoped to agree new measures to "protect our borders with our Five Eyes partners, hitting people smugglers hard".
Mahmood, formerly the justice secretary, was appointed as home secretary in the prime minister's major cabinet reshuffle over the weekend, replacing Yvette Cooper.
Her appointment has been interpreted as Starmer seeking to send a clearer signal that dealing with illegal immigration and asylum is one of the government's biggest priorities, given Mahmood has a reputation within Labour of being a hardliner on the issue.
On Sunday, Defence Secretary John Healey confirmed the government is looking at expanding the use of military sites to house asylum seekers, as it looks to move people out of asylum hotels.
He said officials were also considering other types of "non-military accommodation".