The agency has been charged with regenerating the brand and raising its profile with a multimedia campaign to launch later this summer.
Mother, which opened in the US last year, headed by Red Cell's Andrew Deitchman, won the business following a review involving a range of undisclosed roster and non-roster agencies.
Milwaukee Best has not advertised on TV in the US since 1997, when the Dallas shop Square One created a campaign carrying the endline: "Release the beast."
The brand has since been without a creative agency.
The Mother partner Andy Medd said: "This is another brilliant win for Mother New York. The team over there is winning pitches against the best agencies in the US and we are going to bask shamelessly in reflected glory. Miller is a particular triumph, as it means it has a presence in an important sector. Mother New York is growing stronger with every move it makes."
Sales for Milwaukee Best have dipped in recent months, down 10 per cent at the end of January 2004, compared with the previous year.
A place on the Miller roster could prove advantageous for the agency's London outfit, which is without a brewery client.
Mother New York's other clients include the National Basketball Association and the Denver-based burrito fast-food chain Chipotle.