That Lovell White Durrant is the groundbreaker is not too surprising. Their decision, given Mrs MacDonagh's gender and youth - she is 42 - is, she believes, symptomatic of the firm. "If a thing is right, they will do it. They have no hard-and-fast preconceptions."
They are also successful: partners and staff number more than 1,400 in offices worldwide, including New York, Paris, Prague, Ho Chi Minh City, Hong Kong and Tokyo.
Mrs MacDonagh became a partner in Lovells in 1981, having joined the firm in 1978, soon after qualifying. She began specialising in planning law almost immediately.
"The organisational side appealed to me," she says. "You've got to get under the skin of a business - it's not just pure law." She also enjoys working as part of a team in a large firm. "Lovells is a very nice place to work." Her grooming for managing partner could be said to have begun two years ago when she became head of the property sector and thus part of the firm's executive committee under its new management structure. She also joined the Law Society's council, one of eight women out of 75 members. Clearly, some fee-earning had to go, but she has enjoyed the "nice mixed diet" of work.
Fee-earning will go when she becomes managing partner - a "full-time-and-a-half job". Lovells's system involves two joint managing partners, whose terms in office overlap. Mrs MacDonagh will take over in May from Charles Dodson, and join Michael Maunsell, who has been re-elected for a further two years.
All management functions of the post are vested in the managing partners, including running the eight overseas offices, training and personnel. "We have to be able to carry out each other's roles," she says. "The demarcation of work is not rigid, but depends very much on the personalities of the two people involved. You probably find individual strengths coming to the fore."
She will remain on the Law Society council. "I am really enjoying it," she says. "The society is an awfully big edifice, and there has been a lot to learn. When you get under the skin of the organisation, you see how much is going on there."
It is too early to discuss thedirection she intends to take as managing partner, but she says she is keen on communication. "I want to encourage novel ideas," she says. "It's all too easy to let them slip through the paving cracks. Also, lawyers will have to wake up to the fact that they need to follow new trends quickly, and learn not to be afraid to abandon new markets."
Mrs MacDonagh has three children, aged nine months, four and 10. Her husband is also a partner in a City firm. Marrying career and family has, she says, been a question of following a few simple rules.
"Something has to go," she says. "In my case, it was consulting. I live 10 minutes from the office, which means that I can work until 7pm and begin a family evening at a quarter past. I may have sacrificed the rolling acres, but it was a very positive decision."
An element of luck is also necessary, she acknowledges. Her family has been fortunate with a "good continuity of nannies", and Lovells has been supportive.
"They have let me have three periods of maternity leave while I have been a partner. It's been a two-way process. I had a decent amount of time off. I didn't rush back to work after the last contraction. But if I wanted to be involved in work, that was all right, too.
"One of the things I'm happiest about is that I haven't had to abandon my family or my career for the other. I have a rich family life, but I enjoy coming to the office every day. Children are such a great relaxer. As you walk in the door they require you to focus on them immediately. It's a wonderful mind-clearer."
The ease with which she discusses her family in the context of a managing partnership may reflect the fact that she has never met any discrimination. Is it because she has not expected to find it? "It is easy to jump to the wrong conclusion," she says. "It does happen in some establishments, but I can honestly say I have never felt the glass ceiling at all."
This has also been true of clients: "It is much less unusual now to have women in meetings, and more than half of Lovells's fee-earners are women. But even when I started, after the first vetting most clients turned out to be extremely loyal."
Her approach may have something to do with it. She has a personable and non-aggressive manner, worlds away from a stereotypical City firm manager. She does not see herself as ambitious. "I let things happen. I've always been keen to do well, but I didn'tsee a grand plan. I like doing things for personal satisfaction." But you do need luck on the way, she says.
She is excited about her new job, but doesn't underestimate what hard work it will be. It is the equivalent of running a huge company with eight foreign offices, she says.
"There are 120 people in Hong Kong. It's a task in itself to make sure they're very much Lovells people. I want to make sure that the quality a client expects in London is carried right through the firm."
High achievement relatively early in life begs the question, what next? "My husband asked me that," she says, "but I honestly don't believe you can think more than about three years ahead. There may be a new area of law buzzing by then, but you can't anticipate. If I think back three years, it wouldn't have occurred to me that I'd have had this chance."