Monfardini's competitive career began in karting in the mid-1990s. He finished second in the Minikart class of the Torneo Industrie in 1996 and followed that with second place in the Italian Championship's 100 Junior class and third at the Torneo Industrie in 1998.
Moving into car racing, Monfardini competed in Formula Renault 2.0 Italy from 2001 onwards. He made two appearances in the International Formula 3000 championship in 2003 with Barcelona Competicion, retiring from one race and finishing tenth at Monza. In 2003 he also made two appearances in the Formula Renault 2.0 Eurocup for Durango and raced in Formula Renault 2.0 Italy, finishing tenth in that series with 38 points.
For 2004 Monfardini contested a full season of International Formula 3000 with Spero I.E., driving a Lola B2/50-Zytek across ten rounds but scoring no points, with a best classified finish of ninth at Silverstone. He also made a single start in the Grand Am Series GT class that year with Mastercar in a Ferrari 360 Challenge.
When the GP2 Series replaced Formula 3000 in 2005, Monfardini contested a full season, driving for Durango and subsequently Coloni. He finished 17th in the standings with five points from 20 races, with a notable fourth place in the sprint race at Monza among his results.
In 2006 he joined DAMS for a second full GP2 campaign, completing all 20 races of the season with the Dallara GP2/05-Mecachrome and scoring six championship points to finish 20th in the standings. His best results across that campaign included a sixth place at Catalunya. He did not secure a podium in either GP2 season.
He also made two appearances in the Formula Renault 3.5 Series with DAMS in late 2005 at Monza, finishing tenth and thirteenth.
Following his departure from single-seater competition, Monfardini moved into GT and sportscar racing. In 2007 he drove an Aston Martin DBR9 for Aston Martin Racing BMS across a full season of the FIA GT1 Championship, scoring eight points and finishing 19th in the drivers' standings.
His strongest season came in 2008, when he raced a Ferrari 430 GT2 for Racing Team EdilCris in the International GT Open GTA class. Across 16 rounds at circuits including Vallelunga, Valencia, Spa-Francorchamps, Estoril, Magny-Cours, Monza, and Catalunya, he claimed four podiums — finishes of second, second, second, and third — and completed the year sixth in the GTA class standings and fifth overall in the combined International GT Open classification with 108 points.
In 2009 Monfardini split his season across three series. For Full Speed Racing Team he entered three rounds of the FIA GT1 Championship in a Saleen S7 Twin Turbo, retiring from all three. For Vittoria Competizioni he contested the International GT Open with a Ferrari 430 GT2, finishing eighth in the Super GT class with two podiums — third and second at Catalunya — and finishing 16th in the overall International GT Open classification. He also raced a Ferrari 430 in the Italian GT Championship GT2 class for Vittoria Competizioni, taking a podium at Imola to finish 22nd in the standings.
Monfardini returned to Italian domestic GT racing in 2015, competing in both the Sprint GTC and GT3 classes for Imperiale Racing with Lamborghini machinery across 14 starts, but without reaching the points in either class.
In 2016 he drove a Lamborghini Gallardo GT3 for Cars Engineering in the Italian GT Championship GT3 class. From six starts he took one victory — at Monza on 1 May 2016 — and added a second place at Imola to accumulate 44.5 points, finishing tenth in the standings. He also appeared in the Lamborghini Super Trofeo Europe Pro class that year for Petri Corse, entering two races at Spa-Francorchamps, though he retired from both.