The arrival of McManus after this summer's World Cup in France is another statement signing by the club after already securing a deal for Netherlands midfielder and Euro 2017 winner Jackie Groenen.
McManus will join from local rivals Manchester City, one of City's longest serving players after starting her career in the youth ranks and progressing to the first team in 2009. She left briefly for Sheffield FC in 2013, but returned when the club was re-launched in 2014.
The 26-year-old centre-back won the WSL in the blue half of Manchester, as well as two FA Cups - the second as recently as this month - and two League Cups. Despite her connection to City, she has described the move to the Red Devils as her 'childhood dream'.
McManus didn't actually make her senior England debut until 2018 when Phil Neville took over as manager and saw her potential, but she has been a key player ever since and was a member of the squad that won the 2019 SheBelieves Cup in the United States in March.
This summer's World Cup will mark a first major international tournament for McManus, as she joins new club-mate Alex Greenwood, with England heading to France as a contender.
The pursuit of players at the level of McManus and Groenen, and possibly a handful of others before September, says a lot about what United want to achieve in the women's game next season.
When the club finally launched a senior women's team last summer it was never going to be simply about bowing to pressure and making up the numbers. The plan was for the new team to be successful and compete with the best at home and, eventually, abroad.
So far, so good. United destroyed the competition in the Women's Championship, winning 18 of their 20 games and scoring just shy of 100 league goals. And, having beaten top flight Brighton, Liverpool and Everton in cup competitions, the next season was already looking promising.
The momentum of an incredible campaign and the arrival of even better players to take the squad to the next level makes it an exciting time. The likes of Liverpool and West Ham are expected to strengthen this summer, but United will expect to do well in the WSL.
Pictured: 2018/19 top scorer Jess Sigsworth
Chelsea, Manchester City and champions Arsenal are likely to be a little too far ahead for United to challenge at the very top of the table right away, but a fourth place 'best of the rest' finish ahead of teams like Birmingham would be a very attainable achievement.
Beyond that, things are still only just getting started and the sky is the limit...