Mr Mitchell warned of a race against time to tackle the "devastating famine" in the country and said without action 400,000 children could starve to death.
Two weeks ago, Islamist rebels pulled out of Mogadishu, but conflict has hampered aid efforts in the country.
Mr Mitchell pledged a £25m ($41.5m) aid package, including food and medicine.
His pledge came as the 57 member states of the Organisation of the Islamic Conference pledged $350m to help famine victims in Somalia, after a one-day OIC meeting in Istanbul.
The United Nations estimates that 12 million people have been affected by drought in the Horn of Africa.
Somalia has been without an effective central government since 1991 and has been plagued by infighting between rival warlords.
The UK government says the last British minister to visit Mogadishu, often described as the world's most dangerous city, was Lynda, now Baroness, Chalker in 1992.
Mr Mitchell, who has now left Mogadishu, said: "The stark fact is that in southern Somalia the situation is deteriorating by the day. We could face deaths on a similar scale to those seen in 1991-2 if we do not act urgently now. This is a race against time."
He said more security was needed to protect humanitarian workers - and other countries "must also maintain and increase their support at this crucial stage".
The government says the money announced on Wednesday brings UK support offered to Somalia to £54m since July.
Read more: bbc
