Now at the halfway point in their terms as mayor, the reviews are mixed for Madrid's Manuela Carmena, a former judge and Barcelona's Ada Colau, an ex-housing activist.
They've been accused of missteps, but the disaster some had predicted for the two political neophytes bent on reducing inequalities and austerity has not come to pass.
"Both cities maintain a positive dynamic, they are functioning normally, I don't see any symptom of a 'red' revolution or blunders," says Jordi Alberich, director of the Economy Circle, an influential, non-partisan organisation that scrutinises economic policies and social progress.
End of VIP loges
Barcelona says it has raised social spending by 50 percent since June 2015 while Madrid increased it by 22 percent last year.
"We have eliminated all sorts of sumptuous spending and privileges," Carmena, 73, told AFP.
"This has allowed us to increase social spending a lot while reimbursing our debt," she said.
So for example, the city stopped renting out expensive private property for its municipal services, or paying for VIP spaces at events such as the Madrid Open tennis tournament.
City hall spokeswoman Rita Maestre added as an example that they had also raised taxes on supermarket properties.
According to city hall, Madrid's debt has been reduced by more than €1.7 billion ($1.9 billion) in less than two years. It nevertheless still stood at €3.8 billion at the end of 2016.
Carmena said Madrid had also implemented social measures such as "drastically reducing the price of pre-schools."
But the opposition says the newcomers' inexperience was palpable, pointing to the "disorganisation" of the city hall in many areas.
"They have decided to give priority to early debt repayment before building a single school or library," Begona Villacis, a Madrid councillor for the centre-right Ciudadanos party, told the Huffington Post in January.
"They're reducing debt at a cost of doing absolutely nothing because they're incapable of management."
Both cities have made the right to appropriate housing a focus.
For its part, Barcelona has adopted a series of measures such as building 1,900 units of social housing and aiming for a total of 4,100, or stopping 2,000 planned expulsions of people from homes they cannot afford.
But Gerardo Pisarello, Barcelona's interim mayor while 43-year-old Colau is on maternity leave, says "it's not enough."
Rental prices rose 10 percent last year, and even more in areas popular with the millions of tourists who visit Barcelona every year.
In an attempt to regulate mass tourism so that it doesn't become unbearable for locals, Barcelona has increased its control of unlicensed tourist housing.
It has also slapped Airbnb with a 600,000-euro fine for promoting them, and banned new hotels from opening in saturated areas.
But Joaquim Forn, spokesman for the conservative PD.Cat party, counters that by suspending the opening of two luxury hotels, Barcelona has lost a thousand jobs.
"Their priority is redistribution but not creating wealth, which doesn't make sense," he says.
Both cities also want to be greener.
Madrid, for instance, ordered half of private cars off the roads -- with some exceptions -- in December to fight a particularly bad bout of pollution, a first in Spain.
But this too was met with resistance.
Miguel Angel Belloso, a columnist in the Expansion daily, slammed Carmena's "totalitarian project" and accused her of "persecuting the car."
By Laurence Boutreux with Daniel Bosque in Barcelona / AFP