Mars in 1^st  2^nd 4^th 7^th 8^th and 12^th house from Moon causes  ‘Manglik Dosh’.  Few classics says we have to consider from lagna and kalatra karak venus also. Its been said Manglik should marry Manglik  and may cause troubles if he marries non Manglic. Manglik dosh get cancelled if in similar house placement of Rahu or Saturn and up to an extent Sun. 

there are classical dictum which says any bhava aspecting is on house enriches its effects. then how can for mesh lagna Mars placed in lagna,  or 7^th cause ‘Manglik Dosh’.  Mars in own/exaltation and in friendly house do not cause ‘Manglik Dosh’. Its observed and with experience ‘Manglik Dosh’ should be considered only for mithun and kanya lagna where due to inimical to lagna ‘Manglik Dosh’ can cause harm, consider Mars position if placed in Gemini mithun and kumb rasi res of place It will be less malefic to cause ‘Manglik Dosh’.

In nutshell ‘Manglik Dosh’ is not that serious to be panicked. It requires a bit detailed analysis than just normal whether Mars is benefice or malefic to that lagna, is that any benefice aspects on that shadbala of bhava and strength of Mars itself, and importantly operatively Dasha whether Nars Dasha will be operative at all in horoscope.