As Sunderland was once known for shipbuilding there was only 2 main forms of employment...
shipbuilding by day and stealing from the ships they'd helped build by night.
Apparently there was a saying "they mak'em then tak'em" meaning they build (the ships) then steal from them.
The mak'em bit stuck
and so we are now known as 'makems' or so they say!!?
english.stackexchange.com/users/8358/Leanne
The Wikipedia entry has sourced its answer from the Oxford English Dictionary entry for 'mackem' .
As far as word origins are considered, probably as authentic as it gets.
EDIT : As it seems that everyone cannot access the OED link, here's an excerpt.
There are two possible origins attributed: Probably with allusion to the phrase mack 'em and tack 'em and variations thereof, freq.
said to refer to the shipbuilding industry of the region or.. Perhaps partly also with allusion to the pronunciation of make typical of Wearside, as contrasted with that of Tyneside.
english.stackexchange.com/users/4866/Ankur Banerjee
My uncle was from Sunderland and was clear that it was a reference to the difference in accent which differentiated a Tynesider from a Wearsider.
Geordies would say "maek" and "taek", Wearsiders "mak" and "tak".
Nothing to do with shipbuilding.
There was probably more shipbuilding on the Tyne than the Wear.
I should add that my uncle and my parents were born in the 1920s and knew this usage from childhood.
It's not a recent coinage.
english.stackexchange.com/users/22705/Judi
It may have been a derogatory term at first.
As the people of Sunderland Mak or made the ships but all the money stayed in Newcastle!
As a mackem of today and a proud inhabitant of Sunderland it is a local term we love as it defines us as separate from the people of Newcastle.
The most insulting term you can call anyone from Sunderland is a geordie .
english.stackexchange.com/users/37519/Paul Fletcher
I'm a born and bred Sunderland lad and a proud Mackem.
I can confirm the term comes from the ship-building history as we used to "Mak" the ships, or make the ships.
english.stackexchange.com/users/38456/MackemDave
It comes from when miners, Geordies (Newcastle) called Sunderland makems.
It's also to do with football and the rivalry, but the name "Mackem" came from the mining days and is an insulting word towards people of Sunderland.
english.stackexchange.com/users/32388/chriseth
To just add to the comment about "Mackem" also being related to football, the word effectively seems to have two meanings now: someone who's from Sunderland, and someone who supports Sunderland AFC.
Clearly, many or most Sunderland fans will be from the city anyway, so there's no distinction for those people.
But, someone from outside Sunderland, and who therefore isn't a Mackem in the "traditional" sense of the word, will be referred to as a Mackem if they support that team.
english.stackexchange.com/users/58524/GeordieG
As a child in Newcastle, it was explained to me that the Wearside (Sunderland) ship-builders would vigorously underquote those on Tyneside (Newcastle).
They would make the ships, but run out of resources, and hence the more reputable shipbuilders on Tyneside would take the partially finished ships and complete the work.
Hence "They mack'em and we tack'em".
This is translated as one of the following:
They make them
and we take them
They make them
and we tack them,
i.e. fix them up.
As you might guess, this explanation was given to me by a loyal and wildly biased Geordie, who would never have a good word to say about "the great unwashed" from local rivals Sunderland, so is added more as an aside, rather than genuine etymology.
english.stackexchange.com/users/43961/Phil M Jones
In the north-east of England, if nowhere else, people from Sunderland are called "mackems". Does anyone know why this should be?  Wikipedia  suggests a number of possibilities. Are any of these correct?
Why are people from Sunderland called "mackems"?
english.stackexchange.com
<slang><names>
english.stackexchange.com/questions/11826
english.stackexchange.com/users/305/Brian Hooper
There is no clear reason why people from Sunderland are called "Mackems" although there are a number of theories. One is that the word comes from the phrase "They mack'em and we tack'em", referring to the fact that shipbuilders in Sunderland would make ships and then shipbuilders on Tyneside would complete the word. Another theory refers to the different in accent between Sunderland and Newcastle, and it may have originally been a derogatory term. Sunderland was known for its shipbuilding industry.Apparently, there was a saying "they mak'em then tak'em" meaning the people from Sunderlan built the ships, then stolefrom them. It may have been a derogatory term at first, suggesting the people of Sunderland "Mak" or made the ships but all the money stayed in Newcastle!  It is also suggested that it was the miners, Geordies (Newcastle) who first called Sunderland people "makems". It was also seen as a reference to the different accents in Tyneside and Wear. Geordies said maek" and "taek",  whereas Wearsiders "mak" and "tak". It is a local term which defines them as separate from the people of Newcastle, but they are proud of it, although some say it is perceived as insulting. In modern usage "Mackem" is also related to football, the word effectively seems to have two meanings now: someone who's from Sunderland, and someone who supports Sunderland AFC. Someone from outside Sunderland, and who therefore isn't a Mackem in the "traditional" sense of the word, will be referred to as a Mackem if they support that team.   
The term "Mackems" to call people from Sunderland comes from the ship-building history to "Mak" the ships, or make the ships. it was a reference to the difference in accent which differentiated a Tynesider from a Wearsider. There is no clear single answer to this question, but the term "Mackem" could be derived from the Wearside accent or be related to Sunderland's shipbuilding history.
Hence "They mack'em and we tack'em".
Derogatory term at first. Football connotation. Football connotation
The term "Mackem" may have been a reference to the different accnet on Wearside compared to that on Tyneside. The term may be related to Sunderland's shipbuilding history.