Armstrong signed a three-and-a-half-year contract in the final stages of the window while Mackay-Steven completed his move to Parkhead having already signed a pre-contract agreement
He has signed until 2019.
The move severely weakens United's attacking options just two days after they set up a QTS Scottish League Cup final meeting with Celtic but they have the compensation of about £2million to add to their increasingly healthy finances.
United failed in a bid to re-sign midfielder Scott Allan from Hibernian but another former player, left-back Paul Dixon, returned to Tannadice from Huddersfield as Conor Townsend's loan move from Hull was brought to an end.
Celtic also signed forward Michael Duffy from Derry City and tied up 19-year-old defender Eoghan O'Connell on a contract extension that keeps him at the club until 2017.
Both United and Celtic were linked with moves for Kilmarnock forward Robbie Muirhead on one of the busiest transfer deadline days in Scotland in recent years.
Rangers strengthened their Scottish Championship title quest by bringing in five loan players from Newcastle in a development that is likely to be the subject of scrutiny when Magpies owner Mike Ashley faces a Scottish Football Association judicial panel over his dual interests on March 2.
Slovenia international Haris Vuckic, a 22-year-old attacking midfielder; Swiss teenage defender Kevin Mbabu; 20-year-old English defender Remie Streete; former Coventry midfielder Gael Bigirimana and Northern Ireland international midfielder Shane Ferguson all completed loan moves, reportedly at no cost to the cash-strapped Ibrox club
Their main Scottish Championship rivals both signed midfielders - leaders Hearts snapping up 22-year-old Dutch-born Kenny Anderson, whose father is Scottish, from Belgian side RKC Waalwijk while Hibernian brought in former Aberdeen midfielder Fraser Fyvie after the 21-year-old was released by Wigan
Hamilton had another huge pay day after selling top goalscorer Tony Andreu to former boss Alex Neil's Norwich side for a fee of more than £1million
They also released their second-top goalscorer Mickael Antoine-Curier but failed in a bid to sign Falkirk striker Rory Loy
They did earlier sign 21-year-old Czech midfielder Nico Sumsky following his departure from Slovakian club Dukla Bystrica
Aberdeen earlier made an ambitious move by snapping up midfielder Kenny McLean from St Mirren.
The Dons appeared to trigger a release clause in the 23-year-old's St Mirren contract to purchase the midfielder for a six-figure fee.
Aberdeen, who loaned striker Declan McManus back to Morton, have secured the former Scotland Under-21 midfielder on a three-and-a-half-year contract and also signed 18-year-old former Bayern Munich and Borussia Dortmund defender Lukas Culjak until the end of the season.
St Mirren replaced McLean by signing 18-year-old attacking midfielder Emmanuel Sonupe from Tottenham on loan until the end of the season but failed in bids to sign Danny Swanson and Rangers midfielder Dean Shiels, although manager Gary Teale hopes to sign free agents in the coming days.
Former Dundee United midfielder Swanson later joined St Johnstone on loan from Coventry, replacing the released Gary McDonald.
Motherwell made three youthful signings - England Under-21 goalkeeper George Long arrived on loan from Sheffield United; 19-year-old Everton midfielder Conor Grant also joins on loan; and 20-year-old winger Nathan Thomas signed after being released by Plymouth.
Partick Thistle re-signed striker Lyle Taylor on loan from Scunthorpe following his successful spell at Firhill in the second half of last season, when he netted seven goals
The Jags released another striker, Nathan Eccleston, to pave the way for the deal
Dundee had a busy day, signing striker Paul Heffernan from Hibernian and making Stephen McGinn's recent loan move from Sheffield United a permanent transfer
Dundee also signed teenage defender Ben Priest from Wolves, although he is not viewed as an immediate first-team player, while sending out Phil Roberts and Dylan Carreiro on loan
Roberts has signed for Alloa while Carreiro has rejoined Arbroath.
Meanwhile, Ross County released Dutch midfielder Darren Maatsen
Motherwell added a fourth signing just before the deadline - Kidderminster winger Marvin Johnson.
And Dundee United fans got a much-needed boost when their club signed Scotland Under-19 international Muirhead.
Source : PA
Source: PA