Both teams had players sent off in the first half with on-loan Stags striker James Alabi, making his debut after signing from Stoke this week, and Shrimpers midfielder Conor Clifford seeing red.
The visitors finished the game with nine men after midfielder Lee Beevers was dismissed in the closing stages after receiving a second yellow card.
In between the red cards there were also goals. Anthony Straker struck after 25 minutes and a Alan Marriott own goal a minute before half-time sent Southend into the break in control. Kevan Hurst, once of Chesterfield, completed the scoring in injury time.
The Shrimpers went ahead when Straker tapped home a fantastic right-wing cross from Hurst.
Mansfield suffered another blow shortly afterwards when Alabi was dismissed for a challenge on Luke Prosser.
Southend were also reduced to 10 men in the 40th minute when Clifford was ordered off after making a two-footed lunge at Calvin Andrew.
But the hosts were still able to double their lead on the stroke of half-time when Ryan Leonard's right-wing cross hit the head of Stags skipper John McCombe and was carried over the line by goalkeeper Marriott.
Mansfield's misery was then compounded in the final minute when Beevers, booked early in the second half, received another yellow card for a foul on Will Atkinson.
And Southend wrapped up all three points with a long-range effort from Hurst in the fifth minute of stoppage time.