The Black Cats were minutes away from remaining in the Championship back in May's play-offs (on multiple occasions), but fought back with their 'til the end' mantra to deliver three weeks that many would've considered the absolute high point of supporting their team.
That was followed by what several fans have labelled the best transfer window in the club's history - with 14 incomings, a reported £150 million plus spent and upgrades or reinforcements in quite literally every position on the pitch - the most spent by a newly promoted club, ever.
With the well-documented plight of newly promoted teams in recent years, that level of investment is a welcome change in the Premier League and has so far seen success with two wins out of Sunderland's three opening fixtures.
Here, Flashscore assesses a historic summer for the Black Cats.
An entire team of incomings
With 14 new players viably covering every position on the pitch, Sunderland can effectively put out an entirely new starting XI if they want to.
To get it out of the way, here's the full list of players signed and their reported transfer fees, including add-ons:
Goalkeeper: Robin Roefs (£11.5m)
Defenders: Lutsharel Geertruida (loan, £17m option to buy), Nordi Mukiele (£12m), Omar Alderete (£10M), Reinildo Mandava (free), Arthur Masuaku (free)
Midfielders: Granit Xhaka (£17.5m), Noah Sadiki (£19.5m), Habib Diarra (£30m), Enzo Le Fee (£20m)
Forwards: Chemsdine Talbi (£19.5m), Simon Adingra (£20.5m), Brian Brobbey (£25m), Bertrand Traore (£2.5m)
As you might expect, that sets a record for the most money ever spent by a newly promoted side - but that's a statistic that perhaps needed to be met to have a chance of staying in the Premier League.
Spending was also helped by the sales of Jobe Bellingham (£32m) and Tom Watson (£10m) before pre-season began, providing a substantial amount of wiggle room in relation to the dreaded PSR.
Sunderland broke their transfer record early in the window with the signing of Le Fee, and then Diarra for double their previous highest purchase (Didier Ndong for £13.6m).
That Ndong fee would then be surpassed five more times over the next months, highlighting a fresh impetus to keep up with others in England's top division.
Scattergun or carefully planned?
Sunderland's recruitment might seem a bit scattergun from an outside perspective, particularly with three late signings on deadline day.
The harsh truth is, though, that the Black Cats needed serious upgrades to have a fighting chance in the Premier League - sentimentality favours no one when it comes to survival, particularly not a play-off squad.
And while the bar is low for how some clubs operate in the transfer market, Sunderland's recruitment team has been their biggest asset over the past few years.
Florent Ghisolfi was appointed director of football back in June to better utilise the European markets and assist fan-favourite sporting director Kristjaan Speakman in building a team capable of thriving in the Premier League.
Sunderland scouts their targets well in advance of signing them - a move for Adingra was pursued back in January - while they also react well to market opportunities like Xhaka.
Lots of money has been spent and several new players have arrived, but each addition gives manager Regis Le Bris a new tool to craft out the best possible attempt at staying in the league.
That's not to write off the play-off heroes either, with all of Sunderland's goals so far scored by players who featured heavily last season.
The recruitment team also showed a strong ability to adapt to any unforeseen changes - for example, quickly snapping up Brobbey (who had been linked earlier in the window) to replace Marc Guiu after the latter's loan move from Chelsea was cut short.
Likewise, they pivoted to Geertruida on deadline day after a failed move for Bologna defender Jhon Lucumi.
Can 14 new players gel quickly enough?
The big question remains: can Sunderland's additions hit the ground running, or will it all be for nothing in the end?
Le Bris's side has done a solid job of answering that so far, after two wins from three games that featured at least eight new signings in each starting line-up.
They can't afford to let that momentum slip, and it's crucial to get as many points on the board as early as possible.
It should also be said that it would be pretty unheard of for all 14 players to be a success - it's almost a given that at least one or two will fail to cut it.
What can't happen, though, is having six or seven or more new signings that aren't up to standard.
It remains to be seen whether this does in fact turn out to be Sunderland's best transfer window ever, as a lot of that hinges on Premier League survival.
What can't be doubted, however, is the hope and pride it has ignited amongst supporters. The Wearsiders now have a fighting chance of staying in the top flight.
Deadline day marked 100 days since the Black Cats' historic play-off final win - and not many could have predicted the incredible rollercoaster ride that followed.
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