The specialised ambulance teams halved the time between a patient sounding the alarm and a treatment decision.
Researchers said the teams could improve stroke outcomes, but other academics warned the findings may not be so applicable to rural areas.
Stroke treatment with the thrombolytic drug alteplase can only take place within four-and-a-half hours of a stroke and after confirmation of a blockage with a CT scan.
Delays in reaching hospital mean only 15-40% of patients arrive early enough for this treatment, and just 2-5% of patients actually receive the drug.
Researchers had suggested that by assessing patients at the scene using a mobile stroke unit - equipped with a CT scanner, laboratory and telemedicine link - the diagnosis can be brought forward so far more patients can be treated.
In the study led by Professor Klaus Fassbender of the University of Saarland in Germany, 53 patients were treated by the mobile stroke unit before they reached hospital and 47 with standard care.
Treatment before hospital led to a treatment decision within just 36 minutes on average, compared with 76 minutes in the standard care group.
The mobile unit lowered the time from symptom onset to treatment decision to under one hour for 57% of all patients. Just 4% of those on standard care received this decision within an hour.
Professor Fassbender said: 'The mobile stroke unit strategy offers a potential solution to the medical problem of the arrival of most stroke patients at the hospital too late for treatment... and substantially breaks, to our knowledge, all reported times or stroke management.'
The authors added: 'According to the generally accepted "time is brain" concept, such a large reduction in delay should translate into improved outcome... although in secondary analyses no significant difference was recorded in the numbers of patients who received thrombolysis or in neurological outcome.'
In an attached editorial, Peter Rothwell and Alastair Buchan from Oxford University, said the usefulness of the mobile teams would depend on the setting.
Rural areas would likely fare less well due to delays in the mobile stroke unit reaching the patient, they said. 'Nevertheless, this trial has shown convincingly that in at least some settings a mobile stroke unit-based service is feasible and can substantially reduce delays to treatment.'