The 22-year-old joins the club from Birmingham, bringing with her a wealth of domestic and international experience.
Christiansen expressed her delight at signing for the Blues, declaring that she found the opportunity too good to refuse.
“I’m very happy to have joined Manchester City and I’m now looking forward to starting pre-season with the rest of the team,” she smiled.
“I was very impressed with the developments at the club and their vision for women’s football. There’s a great team here and I am very excited to be a part of it.”
The youngster, who was born in Macclesfield, began her career at youth level with Manchester United before joining Everton. After breaking into the Toffees’ first-team, she went on to make her Champions League debut for the Merseyside outfit during her one-year stint at the club.
In 2010, Christiansen joined FA Women’s Super League side Birmingham. She became an integral part of their team, securing an FA Cup winners’ medal in 2012 and accruing even more Champions League appearances along the way.
In addition to her domestic exploits, the forward can also boast seven years’ worth of international experience, having made her England debut at under-15s level and progressing through the ranks to the under-23s squad.
Alongside new teammate Toni Duggan, Christiansen formed part of the England side that won the UEFA Women’s U-19 Championship in 2009 and received a silver medal the following year.
Earlier this month, she was selected in the senior squad for the first time for the Three Lions’ training camp in La Manga.
She has also participated in the 2008 under-17 and 2010 under-20 World Cups and helped Great Britain to win a gold medal in the 2013 Summer Universiade in Russia.
Christiansen, who is currently studying at university, hopes that her achievements will benefit her new club and admits that her familiarity with the area and with some members of the squad will ease the transition into her new team.
“I know of a few of the girls from playing with England and so that should help me settle in,” she stated. “And I’m now closer to my hometown of Macclesfield, so that will also help.
“It’s a very exciting time and I hope I can play my part in ensuring that the first year in the FA WSL is a successful one.”
City Women manager Nick Cushing was overjoyed to capture the youngster’s signature, declaring: “Isobel is an excellent addition to our squad and we are delighted to have completed her signing.
“She adds to our attacking personnel and strengthens our chances of becoming competitive this season.”
Head Coach Leigh Wood echoed the sentiments:
“Isobel is a player that, despite her young age, has FA WSL and Champions League experience,” he asserted. “She has proven that she can score goals in both these competitions and we are really looking forward to her joining up with the squad this week.”