Guillermo's career began in Argentina, where he wrote an accounting package at twenty and grew it over fourteen years into an ERP platform adapted for municipal governments, pharmacies, grain receivers and meat processors. One city government ran it for three decades. He also helped found and run an institute teaching technology to working adults and public officials.

In the United States he built early mobile payment software: handheld applications with direct connectivity to a major processor, store-and-forward transactions that held offline and sent on reconnect, and municipal parking enforcement delivered by reverse-engineering an undocumented COBOL data structure. He then built the multi-rail gateway, kiosk software and back end behind national wireless carriers' prepaid payments across credit, PIN debit, check and cash, with store-level reconciliation.

Nine years at an Asia-founded payment terminal and gateway group followed. He arrived to run global infrastructure and left as chief technology officer. Within a week of landing he launched a PCI-compliant gateway with live card transactions, which moved the company's strategy beyond terminal hardware into gateway services. He delivered the first mobile printer certified under Apple's MFi standard and led certification across Mastercard TQM, PCI DSS Level 1, PA-DSS and EMV.

He also ran the thirty-day shutdown of a three-hundred-employee factory taken by eminent domain, arriving to find the assigned leader gone and the workforce on strike. Every open order was completed, the equipment sold and the site handed over on schedule, with a farewell event for the people who had worked there.

Nearly ten continuous years at a U.S. merchant acquirer processing several billion dollars a year for thousands of merchants came next, moving from senior vice president of technology to executive vice president of innovation and then of product. Recruited to improve integrations with independent software vendors, he was given the technology organization within his first year. He named single-gateway dependency as the company's largest structural risk and set the direction that became a processor-agnostic orchestration platform: one API into dozens of gateways, hundreds of processors and over a hundred currencies, with gateway-independent tokenization that turned a processor change from a migration project into a configuration decision.

Along the way he stood up eight functions the company did not have, among them project management, sales engineering, integrations and data engineering. He rebuilt the settlement pipelines buried in a multi-million-line legacy monolith while old and new systems ran side by side, turned merchant underwriting from case-by-case judgment into documented decision rules, and led a two-year AI adoption program inside a skeptical engineering organization, won on production results rather than mandate.

Through Lizasoain Consulting he advises payments and fintech companies on settlement modernization, gateway and processor integration, underwriting automation, data infrastructure and practical AI adoption, and runs technology and product due diligence for sponsors and management teams before a deal and the integration work after it. Alongside the practice he builds and runs his own production systems with AI coding agents and agent-driven decision logic, under the same review discipline he brings to clients (how he works with AI).

Across three countries, 26 years in payments and nearly four decades in technology, the pattern holds: find the real constraint, build what fixes it, prove it in production, and hand it to a permanent leader.