Jaschek was born on March 2, 1926, in Brieg, Germany, (now Brzeg, Poland). His family moved to Argentina in South America when he was 11. In 1947, he was hired at La Plata Observatory. His wife, Mercedes Isabel Corvalán de Jaschek, was an Argentine stellar spectroscopist, who also began at the National University of La Plata in 1947. The pair collaborated throughout their lives. Carlos Jaschek received his Ph.D. in astronomy in 1952.
After a year in the United States, he became professor in astrophysics at La Plata University and the director of the Astrophysical Department in 1957. Jascheks initial research at La Plata involved observing minor planets. He began programs in stellar spectroscopy and worked to develop equipment particularly in photoelectric photometry. He was also involved with Argentinas fledgling radio astronomy and space program. In 1972, he organized the first astrophysics conference in Latin America.
From 1957 to 1973, Jaschek made contacts with other spectroscopists, creating lifelong collaborations with astronomers in other nations when he travelled to observatories and astronomy departments including Yerkes Observatory, Perkins Observatory, Ohio State University, the University of Michigan and elsewhere. He was Invited Professor at Perkins Observatory in 1964 and 1967, and also at the University of Chicago (Yerkes) as a research associate in 1967. He was invited professor at the Argentine National Observatory at Córdoba in 1968. In 1970, he was invited professor at Cordoba, Ohio State University, and at Geneva University. Mercedes Jaschek also conducted research at Cordoba, Perkins, Yerkes, Michigan and Geneva.
From 1970 to 1973, Jaschek was the vice president of the International Astronomical Union's (IAU) Commission 45 Stellar Classification, and he became president of that commission in 1973. He moved from La Plata to Europe in 1973 due to the political situation. There was political instability at that time in Argentina.
In 1973, Jaschek was again at Geneva University as invited professor. He was then hired as an associate professor at Strasbourg University. He was at Strasbourg from 1974 to 1993, and he became a French citizen, as did his wife. He later became full professor, and supervised many students.
Jaschek encouraged the development of automated methods for stellar classification classifying  stars. He also encouraged the development of astronomical data bases. He was "a force behind the development of the C.D.S," the Center de Données Stellaires (Center of Stellar Data). In 1968, Jaschek had described the problem of the increasing volume of astronomical data without adequate means of collecting and distributing it:
After a year at Geneva, Jaschek became Director of the Center de Données Stellaires(CDS) in Strasbourg. The CDS, begun in 1972, was an innovative but challenging project to create a central repository for astronomical data, an idea "even questioned by many French astronomers". In 1974, its first director, Jean Jung, changed careers. Carlos Jaschek officially began as Director in 1975, and Mercedes Jaschek strengthened the Centers limited scientific staff. In 1977, Jaschek summarized the strengths and weakness of the worlds astronomical data bases and noted that, with the exception of peculiar stars,  considerable advances were being made. Jaschek noted:
Jaschek directed the Center of Stellar Data (CDS) from 1975 to 1990, As Director, he was "instrumental in organizing world-wide access to astronomical data with special sensitivity to third-world countries. As the second director of CDS, Jascheks 15-year term was said to have brought "spectacular progress for CDS" by increasing its international reputation as it was established as the leading astronomical data base. When it expanded its focus from stars to include other non-solar system astronomical objects, it changed its name from Center de Données Stellaires to the Centre de données astronomiques de Strasbourg' (The Center of astronomical Data of Strasbourg).
As Director, Jaschek was concerned with improving astronomical naming conventions. In 1979 at an IAU meeting in Canada, over 100 astronomers from 15 Commissions met regarding the designation of objects, and before William P. Bidelman gave the report on their suggested reforms, Carlos Jaschek began to introduce the need for them by saying:
In 1986, he listed the 10 different factors for which a star may be named, noting that the variety was for historical reasons and "It is certainly not very rational to have 10 different practices for the same operation", before suggesting some guidelines.
Jaschek was also interested in the Set of Identifications, Measurements and Bibliography for Astronomical Data (SIMBAD). SIMBAD became operational in 1990. In addition to working to make the CDS the world's largest astronomical database, Jaschek helped with the creation of astronomical data centers in China, Japan, India, Argentina and the U.S.S.R.
Jaschek worked in spectroscopy, photometry, and classification of stars and in statistical astronomy. Carlos and Mercedes Jaschek worked on stars, making spectral classification catalogs and atlases of spectral atlases in visible, ultraviolet (UV) and infrared (IR) wavelengths. They worked on the first classification schemes for the ultraviolet spectrum  They collaborated with Yvette Andrillat on stars in the near-infrared, and Carlos Jaschek had a long-term collaboration with the Marcel Golay group in Switzerland to make comparisons between photometry and spectroscopy.
The Jascheks collaborated on Be, Ae, shell, Ap, and other peculiar stars. They produced a catalogue and bibliography of 2,000 Be stars for the period from 1950 to 1970 based on Mercedes Jaschek's survey of the Southern Celestial Hemisphere.
They were the first to introduce the gallium stars subgroup and to discover the ApSi4200 stars. They were the first to find the rare CNO stars which are O- or B- B-type stars where some of the elements C, N and O show spectral lines that are weaker or stronger than would be expected. They also identified the presence of rare-earth elements in stars of Mn and SrCrEu groups.
Carlos Jaschek helped create the Catalog of Bright Stars (fourth edition).  Together the Jascheks wrote The Classification of Stars, which had 20 editions published, The Behavior of Chemical Elements in Stars (1995), and Spectroscopic Atlas for the Southern Sky Stars and other books. Their reference works were incorporated into the Strasbourg Centre of Stellar Data (CDS).
Jaschek also wrote Data in Astronomy in which he noted he became involved with data at La Plata in the early 1960s, when he was compiling a catalogue with a student for their own use and the student suggested publishing it. He authored or co-authored ~250 refereed publications and over 15 books and was noted for "precise observation and careful classification of peculiar stars" using the MK classification system.
Jaschek was Invited Speaker at ~15 international meetings. He helped organize 14 scientific meetings, and frequently edited or co-edited the proceedings. In 1992, Jaschek founded the Société Européenne pour l Astronomie dans la culture' (European Society for Astronomy in Culture), a cultural astronomy organization which holds yearly conferences and is open to all nations.