Mariniuc, a native of Chişinău, Moldavian SSR, started his sporting career at the age of eight under a strict Soviet swimming system. He shortly became a member of the Moldova Swim Club under his coach Luba Pohilenco, and then officially played for the Soviet Union's senior national team before its complete extinction in 1991.
Following the breakup of the Soviet Union, Mariniuc emerged as Moldova's swimming sensation at every level of the competition. He made his official debut for the Unified Team, also known as the Commonwealth of Independent States, at the 1992 Summer Olympics in Barcelona. While he did not win a single medal, Mariniuc successfully made the final, and finished seventh in the 400 m individual medley (4:22.93). He also posted a tenth-place time of 2:03.72 in the 200 m individual medley, trailing Spain's overwhelming favorite Martin López-Zubero, the winner of the B-final, by 0.38 seconds.
Returning home from the Games, Mariniuc viewed his nation as "forsaken" and "hopeless", discovering that some of the swimming pools did not have a water; others were filled with chemicals or locked. Giving up his sport to support the nation's financial scarcity, Mariniuc worked as a part-time taxi driver riding around Chişinău's urban suburbs to earn sufficient rubles, Moldova's main currency, for a living.
In 1993, Mariniuc left his homeland Moldova with a thousand-dollar air fare for Santa Clara, California, where he trained and worked as an assistant club coach for the Santa Clara Swim Club. Recovering from a physical condition back in his homeland, Mariniuc pulled away from the rest of the field to claim the 800-yard freestyle title (8:19.29) at the Swim Meet of Champions in Mission Viejo, California.
On that same year, Mariniuc added another coveted title in the pool to his collection at the U.S. Nationals in Austin, Texas, and later earned a silver medal for his native Moldova in the 400 m individual medley (4:11.96) at the 1993 FINA Short Course World Championships in Palma de Mallorca, Spain, finishing behind Canada's Curtis Myden by 1.56 seconds.
Three years since he moved to the United States, Mariniuc reached his worldwide breakthrough at the 1996 Summer Olympics in Atlanta, representing the Republic of Moldova. Nearly a month before the Games, he earned three pool titles each in the 200 m IM (2:05.03), 400 m IM (4:23.74), and 800 m freestyle (8:10.60) at the Santa Clara International Meet, representing his host swim club.
At the 1996 Summer Olympics, Mariniuc failed again to collect a single medal in any of his events, finishing eighth in the 400 m individual medley (4:21.15), and twelfth in the 200 m individual medley (2:04.11).
Two years later, at the 1998 FINA World Championships in Perth, Australia, Mariniuc pulled off a twelfth-place effort in the 200 m individual medley with a sterling time of 2:04.58. Adding open water swimming to his program, he also placed eleventh in the 5 km (59:06.5), and twenty-first in the 25 km (5:52:28.7).
Mariniuc swam only in the men's 400 m individual medley, as a 31-year-old, at the 2000 Summer Olympics in Sydney. He achieved a FINA B-standard of 4:24.17 from the U.S. National Championships in Federal Way, Washington. He challenged seven other swimmers in heat three, including Russia's 19-year-old Alexey Kovrigin and Colombia's three-time Olympian Alejandro Bermúdez. Entering the race with a top-seeded time, Mariniuc enjoyed a powerful lead in the halfway mark, but fell short on the freestyle leg to register a third-place time and a lifetime best of 4:23.57, a 1.36-second deficit from winner Kovrigin. Unlike his previous Olympics, Mariniuc failed to reach the top 8 final, as he placed twenty-second overall in the morning prelims. Shortly after the Games, Mariniuc announced his official retirement from the sport.
Since he came to the United States in 1993, Mariniuc became an expert in network and systems administration, database management, website development, and security. In 1999, Mariniuc was appointed to be the pioneer and IT director of ReadyGo Inc., a leader in e-learning authoring tools, where he programmed numerous features to implement a customer support process that resulted in the company winning a prestigious Excellence in eLearning Gold Award. A versatile computer and software expert, Mariniuc is currently the vice-president of FastLane Tek, a technology services company based in Sunnyvale, California, where among other support projects, he assist teams to generate and manage timing systems applied in national and worldwide swimming competitions.
Mariniuc currently resides in Santa Clara, California, along with his wife and former swimmer Eve Walton, and two children.