One hundred patients and staff members had to be evacuated after the blaze broke out just before 3pm on Sunday.
A hospital spokesperson on Monday revealed the blaze to be a “suspected arson incident”.
Scotland Yard confirmed a 53-year-old man was detained under the Mental Health Act, by officers called to the scene around 3pm.
The fire took around two hours to control, according to London Fire Brigade (LFB) which confirmed 100 patients and staff had to be evacuated.
Firefighters had the blaze under control by 5.07pm.
The fire was contained within one clinic room but soot and smoke spread to other areas, requiring the entire department to be shut for “extensive cleaning and safety repairs”, before reopening at 10am on Monday.
A spokesman for the Croydon Health Services NHS Trust said on Monday: “We’re pleased to confirm that our Emergency Department is once again fully open following yesterday’s fire that caused damage to a patient cubicle, with smoke and soot affecting many other areas of the Department.
“Our teams, internally supported by colleagues elsewhere in South West London, have worked throughout the night to thoroughly clean and repair the affected areas and we were able to safely reopen our doors to the public at 9.52am to resume full patient care services as normal.
“We would like to thank our patients and local community for all their messages of support and cooperation at this time.
“We can also confirm that police were called shortly after yesterday’s suspected arson incident and a 53-year-old man has been detained under the Mental Health Act.”
One room on the ground floor of the hospital was “one hundred per cent destroyed” by the fire, according to an LFB spokesperson.
